Background steps that led to my current predicament:
1) My desktop PC had a PCIe ssd running windows 10 without problems.
2) My Windows laptop failed and in order to recover lost data from the SATA ssd installed in the laptop, I installed the ssd in the second M.2 slot of my desktop motherboard.
3) After changing boot priority in the bios, I was able to boot into the laptop's windows partition [Both windows ssd drives are installed in the motherboard at this point].
4) While in windows, after booting from the laptop's partition, I can also see the contents of my desktop drive, which in the context of the laptop's windows partition is given the name "D:".
5) I completed a backup of the laptop's contents to an external drive as planned
6) I changed the boot priority in the bios to boot back into the windows on my desktop's ssd - No success.
7) I removed the laptop's ssd from the motherboard and with only the original desktop's PCIe ssd installed in the motherboard, Windows still will not boot.
When looking at D: from within the laptop's ssd (When it is installed) All of the windows directories are there (\users, \windows, etc. ) My understanding is that I should be able to boot from either of these drives, whether they are installed on their own, or co-installed.
What do I need to do to boot into Windows from the original desktop's ssd? What could have gone wrong during this process?