There are several possible causes for these.  One of course is
a bad disk, or a disk that has gone bad.  But one should check the
other options before determining the disk is really bad.

	Dirty power supply contacts have been known to cause these
problems.  Open the machine and clean the power supply contacts on the
ribbon-type cable that goes from the supply to the motherboard.

	A weak or faulty power supply could also be your problem.
Test the power supply voltages (with the hard drive and motherboard
connected as a load), using some sort of Y-connector off the power
cable to the hard drive.  Test the +12VDC and +5VDC supplies with a
meter, and make sure they are within acceptable tolerances.  If they
are too low, intermittent HDERRs will occur.  There are several
adjustment screws on the power supply (marked +5, +12, -12) that can
adjust these values.  Turning clockwise will increase the value, and
counterclockwise will decrease it.  Faulty power can make a good hard
disk or motherboard appear to be bad.

	More commonly you'll find FDERRs in your /usr/adm/unix.log
file.  Every time you format a new floppy disk, you'll get at least
one.  Floppy disks are prone to more errors, especially if you get
those bargain basement brand types.

	TPERRs might appear if you have a bad or defective cartridge
tape block.  If you start seeing a lot of these during your backup or
verify phases, it would be a good idea to re-format the tape and run
another surface check (to check for possibly bad streams, or good
streams that have gone bad).