Unreadable cluster (+ how to do sector checks?)

[Kobboi]

I'm doing a fsck -t on a FAT32 filesystem on a USB disk. The test is still running, but I already get confirmation about a file that I was unable to read properly. The message speaks of three unreadable clusters. Now I'm wondering if this is some sort of filesystem corruption (maybe due to an incomplete write operation and meaning that if I remove the file, the problem is fixed), or that this is the consequence of bad sectors. So I guess my question really is "what exactly is an unreadable cluster ?". Also, are there Linux tools to do sector based checks?

[BradN]

The usual recommended method is by running "badblocks" on the device - this will list unreadable 1KB blocks.

The recommended way to fix these is, in the case of only a couple bad spots, writing over them (the drive will then reallocate the sector from a different spot on the disk), or if there are either many ranges of bad blocks (more than a couple ranges), or many total bad blocks (more than, say, 100), then it's advisable to replace the drive because it may be beginning to fail.

Recent versions of hdparm have an option to directly write sectors (which would be suitable for repairing the bad spots), but I'm not sure of the specifics on how to use this option.