This post is meant to serve as a warning to others like me in the future.
I have had two WD red drives in a Buffalo LinkStation LS421DE, running in raid 1 for quite a few years. I started running out of space, and want to move to a cloud backup anyway (so the raid redundancy isn't as important to me anymore), so I decided to move to just a bunch of drives with no raid array.
Genius me thought "backup the data, shutdown the NAS, pull a drive, boot it back up and delete the array, shutdown and reinsert drive, and I'll have two separate working drives and one will retain all the data".
Turns out this NAS didn't like that. When I rebooted with the 2nd drive reinstalled, everything stopped responding and the NAS went into "Emergency Mode." It took me an hour of messing with it and resetting this & that to figure out that the drives have to be partitioned and formatted from scratch in order for this NAS to recognize them. When I destroyed the array with only 1 drive installed, the NAS repartitioned and formatted that one fine. But it just completely froze up when trying to setup the 2nd drive that still had data.
Hoping to save someone a headache in the future. Just back it up & blow it all up (or buy a better NAS and avoid the headaches altogether).