I've rebuilt 2 faulty SP4's into 1 - essentially swapping mainboard & SSD into another chassis. All good there, tricky but not impossible. 

Now though, I have a problem booting into Windows, & it's killing me. If I let it boot by itself, it'll end up with a black screen - except for once (!) when I got the login screen for about 1 minute before it went dark on me. So I know it's not a hardware issue, it's just Windows & UEFI combining to frustrate.

If I boot from USB recovery disk or into SP4 recovery, it acts as though it has no SSD, except for the Reset option that tells me my hard drive is locked, please unlock & try again. The recommended treatment for that is recboot commands, which I've tried repeatedly to no avail - /scanos always reports 0 Windows installations. I know it's lying to me, I've seen the login page. (Telling SP4's to stop lying is, unfortunately, ineffectual. Cortana isn't available from Recovery.)

Booting from regular Win10 USB install key shows no hard drive available. Diskpart lists only the USB key. 

I'm sure there's an answer - blindingly obvious, simple & elegant - but I haven't found it. Has anyone here been through this "drive is locked, unlock & try again" drama & found the solution that I'm searching for? 

FWIW - i5, 8GB, 256GB. Transplanted into chassis that previously held i5, 8GB, 128GB.