I'm confused as to what your overall ending objective is.
Do you want to end up with both Win10 and Win7, both installed onto SSD and both bootable from there? Do you want to end just with Win10 on the SSD but being able to copy data files from your old Win7 system on HDD over to somewhere on the SSD?
Or do you really want to undo your Win10 install on the SSD and re-install Win7 from scratch onto SSD, eliminating Win10? Note from my comments below that installing Win7 onto newer hardware machines (e.g. Skylake, and possibly Ryzen as well) requires updated Win7 installation media that includes built-in driver support for USB 3.0 (something which was NOT present in the original 2009-era MS retail Win7 installer media). This is easily doable, if you need it.
Will you retain the HDD and eventually delete all partitions currently on it and re-purpose as one (or several) "data" partitions?
From what I've read so far, sounds like you did a Win10 install to SSD without the HDD in the machine. So the new Win10 install went to brand new empty SSD, including creating the GPT System Partition (i.e. "system reserved") which is where Boot Manager lives on the SSD.
Just for information, Boot Manager supports theoretical booting to one or more bootable OS's. So if you had left your original HDD (with Win7 bootable on it) physically present when you did your new Win10 install specifying SSD as the target (i.e. both HDD and SSD were present), then the Win10 installer would have seen the pre-existing "system reserved" partition with Boot Manager and "boot manager menu" inside it, as well as your existing Win7 system partition.
Seeing that, the Win10 installer would have then just added Win10 to be a second bootable Windows to the "boot manager menu" already on the "system reserved" partition on HDD, and not installed its own brand new "system reserved" partition onto SSD (which is what you have now, since you removed the HDD and only had SSD present when you installed Win10 to the SSD). Had you left the HDD be present along with SSD when you installed Win10, you would have ended up being able to target the new Win10 install to the SSD while also retaining your existing Win7 install on the HDD. BOTH your old Win7 and new Win10 would have BOTH been bootable.
When you then booted the machine the two-Windows Boot Manager menu would have appeared, with the newly installed Win10 system (on SSD) pre-selected and thus marked as your new "default" Windows to boot from. Down one line on the menu list would have been your old Win7 system (on HDD), and if you had wanted to boot to it wanted to it you'd just manually arrow-down to that second line and press ENTER. If you simply pressed ENTER on the pre-selected first Win10 line (or waited 30 seconds and done nothing), you would have booted to Win10.
So with both HDD and SSD drives present during install you end up with a dual-boot result. Boot Manager partition and "boot menu" is still on the old HDD, along with Win7 system C-partition, on HDD. New Win10 system C-partition is on SSD. Note of course that whichever Windows you boot to, that partition gets lettered as C by that Windows. All other partitions get other drive letters assigned, D, E, etc., which you can optionally change to whatever you want using DISKMGMT if you wanted to... except that you can't change C.
You could also later have changed which one you wanted to be the pre-selected "default" Windows on the boot manager menu, which would get auto-booted after 30 seconds without pressing ENTER if you didn't navigate with the arrow-down to the other one manually and then press ENTER. So if you wanted to you could have reverted the menu to show Win7 as the pre-selected default rather than Win10, if you wanted to do that. You also could have deleted Win7 completely from the menu, making Win10 your now one-and-only bootable Windows available using Boot Manager and this menu. Note that with only bootable OS present on the boot menu there is no reason for Boot Manager to even present that 1-entry list to you for your selection, so it just boots there automatically by default when you boot the machine.
In summary, had you done your new second-Win10 install (to SSD, which is your choice) while the HDD is still present and thus "system reserved" and "boot manager menu" and the first pre-existing Win7 system all visible to the Win10 installer, you would simply have ended up with a dual-boot system with "system reserved" located on HDD. Both Win7 and Win10 would have been available to you. The BIOS would have still been pointing to the HDD as first device in the boot sequence list, and would go there when you boot the machine looking for the "active" partition (i.e. "system reserved") to find Boot Manager. Then the "boot menu" would have been examined by Boot Manager, two entries found, and the 2-item menu presented to you on the screen, with Win10 pre-selected as the default. You could choose it immediately with ENTER, or let it time-out and after 30-seconds it would have been auto-ENTER'ed, or you could have arrowed down to the second Win7 line and pressed ENTER to boot to WIn7.
However with only one drive (SSD) in the machine when you did the Win10 install, the BIOS automatically makes it first in the boot-device list sequence. So the Win10 installer only sees one physical drive and that it is brand new empty. So a FULL brand-new-from-scratch Win10 install to a brand new empty drive is performed. This is very different process than had you left your HDD with Win7 on it installed when you did the new Win10 install to SSD to be a second bootable Windows. Either approach is perfectly valid, but the results are quite different and how you then boot is quite different.
Since the target one-and-only-drive SSD was totally "empty" a brand new "system reserved" partition is created on the SSD, containing a new Boot Manager and new "boot manager menu" which contains only the single new Win10 item. As long as the SSD is the only physical drive installed, when you boot the machine this drive is the first device in the boot sequence list examined by the BIOS. So the "system reserved" partition on SSD is read by the BIOS, Boot Manager on SSD is found and launched, the "boot manager menu" is read, Win10 on SSD is seen as the one-and-only bootable OS and so it is automatically started. And that's how you get Win10 to come up, in the SSD-only install and operation.
Now, when you put the HDD back in the machine, you now have TWO DRIVES PRESENT. But the BIOS has to put one or the other of these first and the other one second, in its "boot device sequence list". It examines the first one first looking for "system reserved", and if found will use it. If not found it will o on to the next device in the list, again looking for "system reserved".
Since you now have TWO drives in the machine that the BIOS can see, if you really do now want t6o boot from Win10 on the SSD you must get into your machine's BIOS Setup Utility to set that SSD as the first device in the boot sequence list. That will guarantee the BIOS will discover the "system reserved" partition on the SSD and will use its Boot Manager and "boot manager menu" pointing to Win10 on SSD.
If you failed to change the BIOS to place the SSD in front of the HDD, then the opposite arrangement must instead be in place now. So the HDD is examined first by the BIOS to find "system reserved". And sure enough there is one there, namely the original Win7 version of "system reserved" with the old Win7 Boot Manager and old 1-entry "boot manager menu" pointing to Win7 alone. So this Boot Manager should then have automatically booted to Win7, never offering you the choice of Win10 at all.
In this "problematic" (which is just my own opinion) where you actually have TWO separate "system reserved" partitions (each with Boot Manager and a "boot manager menu") on TWO different drives, and you want to boot to one or the other, you must use yet another technique. The BIOS typically has a special hotkey (typically F12) which if you press it repeatedly during the boot process will produce a special interrupt screen that now allows you to override the BIOS "boot device sequence list" and instead manually choose from any available device now shown on the screen to try and boot from. In other words using F12 presents every possibly bootable device physically present, and you can choose whichever one you want to boot from. This allows you to pick a bootable USB or CD/DVD instead of HDD or SSD, etc.
Note that normally you use the BIOS Setup utility to arrange things to work automatically, and never need to use F12 for manual control. So you could place USB first in the "boot device sequence list", optical CD/DVD drive second, and one true bootable drive (either HDD or SSD) where properly constructed "system reserved" and Boot Manager and "boot manager menu" live third. At boot time if you don't have a bootable USB flash drive inserted, the BIOS moves on to see if you have a bootable CD disc inserted, and if not then it moves on to examine the internal drive (SSD or HDD) to look for "system reserved" (i.e. the partition marked with its "active" flag so that the BIOS can recognize it).
You only need to use F12 in a multi-bootable situation if you didn't do the install of your second Windows while the first Windows was fully visible (i.e. if you removed the HDD when you did the Win10 install to SSD), so that you ended up with TWO DRIVES THAT EACH CONTAIN "system reserved". Now you have to manually point the BIOS to one or the other to boot from right now, and that's what F12 allows you to do.
Otherwise, in a genuine desire to end up with TWO bootable Windows systems (which can either be in two partitions of the same physical drive, or in two partitions on two separate physical drives), you would simply cause the Win10 installer to ADD the new Win10 (on SSD) entry to the "boot manager menu" located in the existing "system reserved" partition on the existing HDD. Not a problem. Perfectly normal... assuming you wanted to end up with TWO BOOTABLE WINDOWS SYSTEMS, Win10 and Win7.
Of course if your goal was to only end up with Win10, then removing the HDD before installing Win10 to the now one-and-only SSD drive makes it easier to later boot to SSD and Win10, and delete all partitions on the to-be re-purposed HDD (once you reinstall it in the machine) and create "data" partitions on the HDD.
However in order to ensure that you boot to Win10 from SSD once you reinstall the HDD in the machine so that both drives are now present, you either have to (a) get into the BIOS to set the boot device sequence list to show SSD first, or (b) use F12 to interrupt the machine boot process in order to allow you to manually select SSD as your boot device in order to point the BIOS there to find its "system reserved" partition and Boot Manager on SSD.
Long rant, I know. But I hope it was helpful.
Final question. Is your Win7 HDD from another machine, and you tried putting it into this new machine? Or is this the Win7 HDD you were previously using on this machine?
I have zero experience with Ryzen. But I do know that with Skylake and newer chipsets Win7 cannot be installed or operate without manual addition of the now required Intel USB 3.0 driver. This new hardware supports USB mouse and keyboard through the "embedded controller" in the hardware, which requires USB 3.0 rather than USB 2.0. Since Win8.1 and Win10 come with USB 3.0 support built into the installer and OS, there's no problem installing Win8.1 and Win10 on the newer hardware (and Ryzen may be in this group).
But Win7 didn't come with built-in support for USB 3.0. So if you want to install Win7 from scratch on a Skylake (and possibly Ryzen) machine, since the install requires mouse and keyboard you must first create brand new Win7 install media with the USB 3.0 drivers added. This is a well-known issue and the solution is easy, if you need it.
Similarly, trying to use a pre-existing previously installed Win7 system from an old machine (older than Skylake) on a new machine (Skylake, and possibly Ryzen as well) then you must first have added USB 3.0 drivers to it if you haven't previously done that. I don't know if your Win7 HDD is an old Win7 from another machine and which might not have USB 3.0 drivers present, or is it actually from this very machine you're now working with so that presumably it must have had USB 3.0 drivers present?
Anyway, I have presented several initial questions for you to answer and clarify. Most importantly,please describe your ending objective if you had both HDD and SSD installed and without any problems, what would you be wanting to end up with?
What machine was the HDD and its Win7 being used on?
Did you have one old machine with this HDD and Win7 and you're migrating to a new machine and want to be able to use the same exact HDD with pre-installed Win7 on it, without doing a fresh Win7 reinstalle?
Or are you trying to upgrade an existing Win7 machine to now use Win10 on that same machine, instead of Win7?
Or are you wanting to have the option of running both Win7 as well as Win10 on this new machine?
