Don't leave a disconnected hard drive next to active
Doing so might damage your offline drive. I've learned it hard way. The heat from active drive might damage the mechanics of the other drive where it would have all kind of errors and failures after reconnecting it. Some times it might work after cooling down other times it might not.
Since I reconnected the drive i found it wasn't behaving correctly, trying to seek again multiple times, had grinding noise on start and os was unable to boot due to errors. After a while I reconnected it again after it cooled down then it finally started working again. I was checking the primary drive while the other was offline at the time.