Wich drive does RAID 1 sync?

Hi!
We upgraded our server from 18.04 to 20.04 last week and we removed one of the drives from the RAID 1 to make an image before starting.
So it's out of sync.

I just want to know if I put it back if it'll sync to the currently running OS?
Not sure if I can hotswop it.

Should I first format it perhaps?

I just really don't want it to sync the old OS back to the drive

All RAIDs I know of, md included, treat re-adding a drive in an expected manner - sync to it, not from it.

Note the reference to "re-adding" - not adding.
Hopefully you have a bitmap, so the whole experience will be a lot less painful.

Well I'm pretty sure I did it the wrong way but it worked. XD
I just formatted the drive that was out of sync, and readded it with sudo mdadm /dev/md0 --add /dev/sdb1
It synched and everything is fine.

It did however cause our ERP server to randomly send out all emails it sent the last month. No idea why.