Thanks for the pointers. I did not see anything like an emulated disk, but I think I might be doing what you suggested
I unassigned one of the 3 data drives, it started showing up in unassigned drives. I started the array. The drive in unassigned section was not mountable, only had format button. I didn't touch that
I then stopped the array, put the drive back into its array spot and started again. So a rebuild is now happening with other 4 drives being read and the one I moved around being written. I think I understand your point reading the other thread you linked - partitions aren't protected by parity, so if the data is intact, it will be recreated on rebuild and while doing so unraid will create partitions fresh. So if data was OK, this might work.
So how do I know that it worked after first disk? It will not say unmountable? and I will be able to see partial data in shares from whatever was on it? If it shows up empty or still unmountable, it didn't work I guess
Curiously, while the rebuild is happening, the drive being rebuilt has gone from "Unmountable: unsupported partition layout" to "Unmountable: wrong or no filesystem". This is expected?
If it does work, since I have 2 parity drives, would I be able to rebuild the the other 2 data drives in a single rebuild rather than one by one? 1 rebuild is about 12 hours for me