I have a RAID 1 mirror on a Lacie 2 Big USB3. When I break the mirror by pulling out one drive, keep making changes on the remaining drive and finally put the first one back in, the drives stay happily out of sync. The unit doesn't seem to detect the mirror was broken. The manual only explains the steps to take to replace a failed drive with a new drive. Does it only re-mirror if the replacement drive is new? What does it look at to determine that?
Taken from Lacie’s support FAQ:
I don't know this drive specifically, but most consumer-model RAID enclosures use very simple logic to determine the health of the array. Usually this is just some metadata stored in the slack space of the disks themselves.
The simple logic only triggers a RAID fault in the following conditions:
That's it. Under "normal" conditions (i.e. you use it the way you're supposed to), replacing a bad disk would trigger a rebuild because the new disk would not have the RAID signature on it.
In your case, since you removed one of the drives while the unit is powered off, none of these fault events gets triggered. When you power it back on with only one drive, the RAID metadata gets ignored (the unit operates in single drive mode). When you put the second drive back in and power it on, the enclosure sees the metadata on both disks is identical and assumes that everything is fine. There is no hashing or checksumming that takes place to determine if the disks are in sync the way a real RAID controller would.
Lucky for you, these "dumb" RAID enclosures also do not do read balancing the way a real RAID controller would either. Data is always read from one disk in the array and the other is basically a live backup, so you're not totally corrupt when you power it on. Make no mistake, though, your mirror is broken and the disk you used on another computer is screwed up. What would happen if you attempted to fail over to it is anyone's guess. The disk should be considered totally corrupt at that point even if another computer says it's fine.