Pending reallocated sectors

If a disk is accessed by mdraid and it gets a read error it will immediately go to the other disks to get the data and afterwards it will write that good data back to the disk with the failed read. So if a user/app reads data there shouldn't be a pending sector for very long (sub seconds). But since the disk has pending sectors I can only imagine they came from the disks own media scan (which doesn't know how to correct them using other disks). I think a disk scrub would cause them to be corrected since scrub reads everything, but there is a regular weekly job installed on debian "checkarray" which is not present on readynas. I'm just wondering why that is not enabled (the command is present in the filesystem).

My pending sector count is 61 and my reallocated sector count is 0 right now. I'm going to order a new disk. I'm just curious why netgear took out the checkarray cron job. And I'm wondering whether there is some other way to get pending sectors with reallocated sectors zero than the disk background media scan. By the way there are no md errors in my journalctl log so I don't think these errors were encountered via md which is consistent with them not being corrected.

thanks
steve