ReadyNAS: lost data after removing a drive : case #17927967

Disks can and do fail at any time. Now the NAS will treat removing a disk as if it were a disk failure, then reinserting the disk will wipe that disk and a resync will take place. At this point you are vulnerable to problems as there is no RAID protection. You'd fact the same issue if you were using another brand. Now, support should be able to determine what the problem actually is but I reckon if your friend hadn't removed and reinserted the disk you wouldn't have come into a situation where this thread needed to be started.

Don't get me wrong, RAID does provide some protection for data, but due to its vulnerability to things like accidental file deletions, multiple disk failures, problems while the array is non-redundant, fire, flood, theft, etc. it should not be considered a backup.

High-availability or redundancy is designed to maximise uptime and minimise the likelihood of needing to restore from backup. RAID and backups are complementary not substitutes. Any experienced IT admin should tell you that RAID isn't a backup.