Too bad you didn't have the manual for Diskey - it told you how to create bad sectors. You slowed the RPM of the drive down to about 230 (288 rpm was the norm, Diskey had a drive speed utility in it) - then wrote a test sector, the trick was to have the drive slow enough that it would write the bad sector successfully but couldn't read it back. 220 was too slow but sometimes 225 worked it depended on the 810 drive.
Diskey was a great program for hackers, when code started to get obfuscated you could apply a mask to the information on the sectors and read the text/opcodes you wanted to change directly vs. hand translation.