Reallocated sectors = data loss?

X-RAID2 is like RAID-5.

I would remove that disk. It's clearly failing and only going to cause problems.

There is a possibility you could have a problem e.g. from another failing disk. RAID isn't a replacement for backing up your data. See Preventing Catastrophic Data Loss

After your volume is redundant again, I would enable disk scrubbing monthly and the Online Filesystem Consistency Check weekly. These are good as they can help problems be detected before they become serious.

Ok I will pull that disk. The next step from tech support was to replace with one they are sending so.

I understand RAID != backup. Thats a good article though, amusing too :)

I have a reasonably recent backup of the important data. I'm interested to understand if any data on the NAS could be corrupted as I'd prefer to keep treating the NAS as the master copy but I don't want to roll over the existing backup with any corrupt data.

Is it fair to assume that *if* the remaining 2 disks are healthy no data should be damaged? I would have thought so, but again, tech support seemed to be suggesting that is not necessarily the case. I don't quite get that since RAID5 should imply total reduncancy of any single disk but maybe there is something more subtle going on with XRAID2 ?

Although its hot swappable I presume its generally better to power down for disk swaps? (I have no pressing need to keep the NAS powered on).

If the other two disks are healthy then most likely the data will be fine. However there are some problems you may possibly encounter. The maintenance checks help reduce the risks of encountering such a problem. RAID is good but it isn't perfect.

Filesystem corruption and issues with parity are possible. If you encounter these you can work with support on the issue.

Thanks for all your responses mdgm.

FWIW the other two disks show 0 reallocated sectors so hopefully it will be fine.

If the reallocated sector count is lower than the number of sectors reserved for that purpose you won't have serious problems.
If the number of reallocated sectors is bigger than that, but there's enough free space available, than you most likely don't have serious problems regarding immediate availability of data on that hard disk.
Reallocating damaged sectors is a failsafe measure and while it definitely is signalling a serious problem with the disk, in most cases there is no data loss at all. It might be that a damaged sector was still readable but no longer writeable or if the sector was really no longer readable there's a good chance this can be handled on application level (i.e. a word document could be repaired or missing information could be guessed by the word application when opening it)
The data from a defunct sector is still preserved by copying it to a spare sector reserved for such purposes, thus "reallocated sector", and in most cases the sector is not completely faulty but maybe one bit flipped, which is usually easy to detect by the application and is then fixed.

The hard disk is broken, needs to be replaced ASAP and will definitely cause problems at some point, but it's very likely that such problems don't yield catastrophic data loss if detected early enough.