In my professional opinion, RAID rebuilds should not be used to do backups this way, if that's what is actually taking place here (check if there's other software installed that is handling the sync, and as others said, post a screeny of Disk Management for us to inspect).
If you are relying on RAID rebuilds to replicate data:
One of these days something truly tragic is going to happen - you're not going to notice that the portion of the mirror that you leave in the system is already failed, and cause an outage, or worse, you're going to reuse an existing backup drive, the RAID controller is going to mistake which drive has the hot copy of the mirror, and you're going to do a rebuild onto the production drive of stale data.
At 20 users, its time to get a proper LTO tape drive (LTO-2 would suffice), and some mainstream backup software like Backup Exec. Reason for going this route is if the truly craptacular occurs, and your office burns down, you can go to almost anyone's small-medium sized shop and borrow/beg/steal their tape drive and software to recover your data, since everyone and their brother runs LTO and Backup Exec.
Just my opinion based on nearly 15 years of enterprise and SMB systems administration.
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-Scott
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