What do these disk errors in syslog mean?

Solution 1:

sd 0:0:0:0: [sda]  Add. Sense: Unrecovered read error - auto reallocate failed

Looks like the drive has bad sectors and is unable to reallocate these (possibly because it's run out of spare sectors). The output of smartctl -a /dev/sda would give you more information on the state of the drive.

Solution 2:

Lassie's saying "arf! arf arf! arf!". Which is dumb, because this has nothing to do with Timmy or wells. This is why you don't take sysadmin advice from dogs.

The drive is giving you an "Unrecovered read error - auto reallocate failed", which basically means "I tried to read, I failed, I tried to recover (read the sector a few more times, apply some ECC, and move the data to a sector that isn't broken), and it didn't work". This probably means (as mgorven says) that the disk is chock full of reallocated sectors already, because the disk's been dying for a while, but I also think it can mean that it wasn't able to recover the sector at all (repeated reads + ECC failed to get a good-looking data block).

Either way, yeah, the drive's very, very cactus. Your data isn't looking real healthy, either.

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Solution 3:

I know this is old, but just in case someone is still reading this post: "DD will also try to read the broken sector(s)" - gddrescue is useful here. It doesn't (okay, it does, but only once).