I was fixing my friend’s computer hard drive whose partition structure had gone corrupt. I restored the partition structure using testdisk. However, on booting through this hard drive, BSOD appeared. It said UNMOUNTABLE_BOOT_VOLUME. The hard drive was perfectly healthy, so the error looked a bit out of place. I looked into google and found that most of the people who had this error had 2 SATA drives. My friend had 2 SATA drives too.
So, I switched the cables (connected HDD1 with HDD2’s cable and vice versa) and voila! It worked!
If that doesn’t work for you, you can remove the other drive completely and see if the problem is really with having 2 drives. You can also try replacing the cables with new one.