Not sure what to make of this behaviour, but here goes.
I have 3 Seagate 4 T drives in a windows 2012 r2 server in a software raid 5. Recently, I logged onto the server to see that 2 disks (specifically the disks in sata slot 1 and 2) were deactivated. I reactivated the drives, let the array sync, then ran all the tests in seatools which came up with no issues.
Just logged in today and found the same 2 drives are deactivated again....
I have begun resyncing, but I'm wondering if there are any better diagnostics to run to see if I need to replace the drives now. Long term I'm planning to buy a nas and move the data to it instead of the windows server, but I wasn't planning to do that for a few months.