Is your raid rebuild failing?
If you have logging, are you getting timeouts on drives during the rebuild?
Have you swapped in another new drive and are still getting timeouts?
Check for vibrations in the server. I recently had a case where a raid failed to rebuild and had these issues.
It turned out to be a muffin fan on the drive backplane that was oscillating, but not when I paid the server a visit to feed it a new drive.
After giving up due to tight time constraints, I restored files to another spot.
After the fire drill, I decided to fire up the old server.
Wham, the fan started making Tusken Raider sounds.
Long story short, disconnected the under-used fan, and the raid rebuilt fine.
Thanks Opsmonkey - or should I say Andy Miller in the comments, for this tip.