I have a NAS appliance that is just over a month old. It is configured to email me alerts generated from the hard drives' SMART data. After one day, one of the hard drives reported that a sector had gone bad and been reallocated. Over the first week, that number climbed to six total sectors for the hard drive in question. After a month, the number stands at nine reallocated sectors. The rate definitely seems to be decelerating.
The NAS is configured with six 1.5 TB drives in a RAID-5 configuration. With such high capacity drives, I would expect a sector to fail from time to time, so I was not concerned when the first few sectors were relocated. It bothers me though that none of the other disks are reporting any problems.
At what rate of relocations, or total number of relocations, should I start to get worried for the drive's health? Might this vary based on the capacity of the drive?