Volume dead after replacing other disk

While that was probably not the best way to do what you wanted, I suspect made no difference.  It is best to swap a drive or insert a new one with power on, not with power off, as it helps the OS determine what the next step is.  Then wait for that one to complete re-sync before inserting another.  But since the NAS does seem like it figured out what to do, started it, and failed during it, it appears that drive 2 simply failed during re-sync, which can happen because re-sync puts more strain on the drives.

ATA errors are not always the fault of the drive.  In fact, they often are not.  They can even result from a problem with another drive that shares the controller, so drive 2 may have been the problematic one all along, or it could be the chassis.

Since the RAID is non-redundant while syncing a replacement drive, you should always make sure your backup is up to date before making a replacement.  Then, if something like this does happen, you can at least recover by factory default and recovery from backup.  But since you have come here, I suspect you didn't do that.

The first best step is to test Drive 2 and the old Drive 3 with a PC using the manufacturer's tool.  Whether or not there is any access possible will help you determine your options.  Also, do you at least have a partial backup in case some, but not all, files are recoverable?