The following is taken directly from the 1581 Toolkit Chapter 9 of the 1581
DOS Reference Guide Copyright 1988 David W. Martin

1581 Bugs

Soon after it's appearance on the market the 1581 disk drive was plagued
with several unfortunate hardware problems that Commodore has since
corrected.

Extensive testing of the 1581 was done and three manufacturing problems were
corrected.

The first one involved an improperly grounded jumper (J1) on the PC board
inside the 1581 disk drive.  Allows the drive to use the 6 ms step rate.

The second involved a bad batch of WD1770 chips that were later replaced
with the WD1772 chip.  A typical symptom of this problem is a corrupted
directory after a disk copy.

The third involves a bad solder connection of pin 10 on device U10.  It
causes intermitant (sic) "device not present" errors.

There is a public domain program to detect the problems.  1581.DIAG is for
the C128 and the 1581.DIAG.C64 of course is for the Commodore 64.