Boot and System installed on two different drives

Hi
I had a setup running W7 with an SSD(M4) and HDD(WD). I had an earlier installation of windows on the HDD, but the new installation of windows 7 has worked flawlessly, until now.

I invested in an EVO 840 and decided that I wanted to install W8.1 on it, and due to some problems with the Sata-ports on my MB I had to unplug my HDD. Which was fine, but I wasn't able to boot my W7(installed on the M4) without the HDD being present. I didn't take notice of that and booted from my USB and installed W8.1 on the Samsung.

Now it's messed up. If I plug in both of the SSDs they both boot windows 8.1. The samsung should boot windows 8.1 and it is, but when I try to boot W7 from my other SSD it opens with the OS installed on the Samsung SSD. If I unplug the samsung the old SSD won't boot. The same goes if I plug in the HDD and the old SSD. It only "wants" to boot windows 8.1 off of the new SSD.

Can anyone shed some light on the matter? It's quite anoying and I've been trying to fix it for the past 4 hours, without luck. I'd like to keep my old windows 7 installation, and it's only as a last resort I'd be willing to reformat that drive. The W8.1 on the other hand, it's only a matter of the installation time!

In advance thanks!

I have included an image of my disk management, please note how the BOOT and SYSTEM sector are installed on different drives.

EDIT:
After a reboot the old SSD doesn't show up in Windows Explorer...

Hi there! If you successfully installed both OS fine, then it could be on the booting order. You need to change the booting order on the BIOS, if you want to run the HD with the windows 7, place it on the top of the list, and if you want the windows 8, also place it on the top.

And also you may want to assign a letter on your Disk 1 which you labelled as old.

Unplug all except Disk0, boot the Win7 installation media or Repair CD - System Repair Disk to run Startup Repair - Run 3 Separate Times until Win7 starts and holds the System Active flags.

Now you can plug back in the other drive which already has its System boot files as signified by the System label on it. Boot it using the BIOS one-time boot menu key. If it won't start unplug all other drives and run 3 Startup Repairs on it, or if it's Windows 8 run Automatic Repair - Run in Windows 8[1]=Performance%20Maintenance.

If booting via the BIOS isn't to your liking install EasyBCD (click Download - no Name or Email required) to Win7 to add the other OS on the Add OS Entry tab.

gregrocker said:

Unplug all except Disk0, boot the Win7 installation media or Repair CD - System Repair Disk to run Startup Repair - Run 3 Separate Times until Win7 starts and holds the System Active flags.

Now you can plug back in the other drive which already has its System boot files as signified by the System label on it. Boot it using the BIOS one-time boot menu key. If it won't start unplug all other drives and run 3 Startup Repairs on it, or if it's Windows 8 run Automatic Repair - Run in Windows 8[1]=Performance%20Maintenance.

If booting via the BIOS isn't to your liking install EasyBCD (click Download - no Name or Email required) to Win7 to add the other OS on the Add OS Entry tab.

I will try your method - But, Disk0 is the one with W8.1 (and which can boot without the system partition, no idea how though)
Disk0
Can boot alone, New SSD, W8.1 working standalone

Disk1
Gives W8.1 Startup Error (no installment of W8.1 on it), Old SSD, has a working W7 installation which I cannot boot via, only willing to boot with Disk0 plugged in, and then booting via Disk0

In that case unplug all but DISK1 to run Startup Repair - Run 3 Separate Times until Win7 starts correctly. If it won't start after 3 separate repairs, then everything is here for Troubleshooting Windows 7 Failure to Boot leading up to if required rescuiing your files to reinstall.

I'd also unplug all but DISK0 to run Windows 8 Automatic Repair

Once both boot independently, set preferred to boot first in BIOS setup, trigger other using one-time BIOS Boot Menu key given on first screen.

If this isn't to your liking install EasyBCD (click Download - no Name or Email required) to Primary OS to add other on Add OS Entry tab.

gregrocker said:

In that case unplug all but DISK1 to run Startup Repair - Run 3 Separate Times until Win7 starts correctly. If it won't start after 3 separate repairs, then everything is here for Troubleshooting Windows 7 Failure to Boot leading up to if required rescuiing your files to reinstall.

I'd also unplug all but DISK0 to run Windows 8 Automatic Repair

Once both boot independently, set preferred to boot first in BIOS setup, trigger other using one-time BIOS Boot Menu key given on first screen.

If this isn't to your liking install EasyBCD (click Download - no Name or Email required) to Primary OS to add other on Add OS Entry tab.

That's what I figured! I'll let you know if it solves the problem when I find the time to try it out!
Thanks!