ReadyNAS X6 with repeated disk failures on same channel

First, ensure you have a good backup.

cdeel wrote:
I did find quite a bit of dust in the enclosure after the first disk failure, and cleaned it out thoroughly at that point. (The machine had, however, been moved from an open-air environment to a running in a closet shortly before all this happened. I never got any high-temperature warnings or anything, but it's worth noting.)

Right.

cdeel wrote:
The replacement disks were both used disks (not unformatted). (I assumed the NAS would overwrite any previous formatting with its own - right?) I think one was previously part of a different ReadyNAS volume, and the 2nd replacement disk I tried was previously used on a Windows machine.

It should usually but the X6 does not support hot-swap. Hot-swapping is recommended to ensure disks get wiped properly. You may wish to hook one of the possibly "good" drives up to your Mac and seeing if you can delete the partitions on it (use "dd" via the Terminal to wipe the partition table), then power down, and insert it into the NAS and boot the NAS up again and see how it goes.

cdeel wrote:
So you're suggesting it may be coincidence that I tried 3 bad drives in a row on the same channel, rather than a problem with the NAS hardware?

NAS hardware would probably be more likely but best to try to rule out a different problem. There's some good troubleshooting steps here (though perhaps not strictly what you're encountering there are some things worth checking mentioned there): http://www.readynas.com/kb/faq/hardware/a_newly-inserted_hard_disk_will_not_add_to_the_raid (note that as your unit is out of warranty you'll need to work on the forum on this issue).

cdeel wrote:
I will try to figure out how I can run SpinRite (difficult as I use a Mac laptop and don't have a real PC box around) or try a brand-new drive. How important is it that I use the same model/size as the other disks on the array?

You can try a larger capacity drive. Best to choose a drive on the compatibility list for e.g. the NV+ (not the NV+ v2) as the NV+ (v1) is a Sparc ReadyNAS like the X6.