Seagate external

So you were able to boot into Seatools on your desktop and it can see your external drive. Right?

If so persist and try to run the short test again, if you get any error message please note it and reproduce it verbatim or post a screensnap.
(As far as UEFI is concerned, I am also not very comfortable and on a few occasions I had a UEFI laptop, I used to disable secureboot and make it legacy boot whenever I wanted to boot from an USB device. But this setting and where you get it may vary.)

Anyway get on with your desktop only and wait for DavidE. I shall only be watching on the sidelines since he has more grip on it.

We may be missing from time to time due to other exigencies. Similiarly you may also take a break and continue when you get a response from the present handler.

Post the error message verbatim and wait for DavidE.

OK, I shall now go ahead and delineate the route I shall take.

Plan A: To run bootice and manipulate Sector 0 – after saving it -, if bootice can see the external drive

Plan B :later if Plan A fails.

1.Download Bootice from Bootice Download - Softpedia.
the 32 bit version if your system is 32 bit and the 64bit version if your system is 64 bit. But stick to one system on which you will do this.

2.Create a folder named bootice on your C: drive and extract the BOOTICE*.rar into it.You will have one single BOOTICE*.exe file in it.

3.Plugin your external drive before you run bootice.

4.Run bootice, select your external drive from the drop down arrow and click on Sector Edit. Please see my post here https://www.sevenforums.com/hardware-devices/310295-lost-partitions.html#post2584426 If your external drive is not seen by bootice end of this plan. Close bootice.

5.If you can see sector 0 : a) post a screenshot b)save it as sector0.bin. By default it will be saved in the same bootice folder. Check the folder and make sure it has been saved. Read the above thread further on how to save it. c) Click on the Partitions button and show what is seen, - only that info box . If you cannot snap it, note it and reproduce.

At any point you can close the windows and exit bootice and rerun later. But always make sure that you have selected the external drive and not the system drive.

Please wait for DavidE before you do this and check whether he would like to continue with Seatools or recommend Paragon Rescue kit. ( Since this was hanging on my head for long I wanted to post this and retire for the day peacefully. And why is your time being shown as US time? Please correct it as Greenwich time in User Control panel.)