I've gotten this error enough to bother posting about it, because I've come across so many servers where this happens, so what could "Error 28" possibly mean? Is your database corrupt, or is this a sign of a RAID failure/corruption or even worse, bad blocks on a clients system who has no RAID and never took backups?
No, check your free blocks, it simply means you have no space. This was the result of a script that was overzealous and backed up the entire database each night, it doesn't take long until the whole partition gets full.
Now that you know this is the likely culprit, have fun clearing out the junk and hopefully you have items that can be deleted.