One thing I just learned the hard way - Don't let Windows 10 perform a start-up repair while another bootable OS (in this case Windows 8.1) is present or it will probably sabotage it.
On my laptop, I was trying Windows 10 on a second SSD in an ODD bracket.Â When I used Windows 10 earlier today, it did an update that resulted in a BSOD on boot.Â After the second failed boot attempt, it then performed a start-up repair and booted fine.Â
When I later tried booting back into Windows 8.1, it showed the animated circle, followed by a black screen and went no further.Â The Windows 8 start-up repair said it could not fix my computer.Â
When I booted back into Windows 10, I noticed that I could no longer access the Windows 8.1 drive.Â It seems like when Windows 10 did its start-up repair, it wiped the security permissions off the Windows 8.1 drive.Â I initially tried adding the 'Everyone' with 'Full Control' drive-wide, but Windows 8 then gave a BSOD with kernel security error, so it seems to require specific file/folder security permissions to boot.Â
Unfortunately from what a fair amount of searching the web, there's no way of restoring the default Windows permissions without doing a refresh/reinstall, so ended up having to go through that process.Â
So for anyone else running Windows 10 on a second drive, don't let Windows 10 perform a start-up repair without first disconnecting any other bootable drive.