I have a ReadyNAS 104 with 3 X 4TB drives installed running RAID 5. It started to respond sluggishly recently and logs showed increasing ATA errors on disk 3 and that it was possiby close to failing.
I purchased a replacement plus one more for the spare bay, swapped 3 for a spare and also installed the 2nd spare at the same time (mistake?).
Booted the NAS which started to flash that data was degraded and then began to rebuild the data once it had picked up on the new drive(s). After around 10 minutes it started to flash an error - Disk DEAD and appears as though disk 2 is not responding.
How can I resolve this? Can I replace disk 3 for the original disk 3? Will that give me time to retrieve the really important data? At the moment I've powered it off and I'm too scared to do something that will damage the data even more.
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?
