What is checksum error?

[Ponos2D]

Hi!

When I try to delete some files, I get message
like:
hd3 has a checksum error in block 253392.
and I can't delete files

I have three months old 40 GB HD Western Digital,
and I'm useing SFS but with FFS gives same error

What is checksum error? Can I repair that somehow,
or is it bad disk?

Thanks!