This error message, and messages like it, almost always indicate a hardware error with a hard drive.
This commonly indicates a hard drive defect. The only way to avoid further data loss is to completely shut own the system. You must also make sure that whatever data is on the drive is backed up, and restore it to a non-defective hard drive.
This error message may also indicate a bad connection to the drive, especially with home brew systems. If you install an IDE drive, always use new ribbon cables. It's probably is a good idea with SCSI drives, too.
In one instance, this error also seemed to coincide with a bad ground between the system board and the chassis. Be sure that all electrical connections are clean and tight before placing the blame on the hard drive itself.
[Peter Moulder, Theodore Ts'o]