"Rick Balkins"  writes:

> Another additional thing is PRML or extended PRML (EPRML).
> The zip disk would resemble a hard drive more then a floppy disk in the 
> encoding scheme.

We might guess RLL. I found a paper from Quantum
(http://www.forensicswiki.org/images/c/c6/Maxtor_PRML_Read_Channels.pdf)
where they say they first put PRML in their Empire drives in 1993. 
Maybe IBM did it already in 1992 when they came out with the first
3.5" 1 GB drive
(http://www-03.ibm.com/ibm/history/exhibits/storage/storage_chrono20.html).

The Empire models were apparently 540 MB - 2 GB in size. The 540 model
had four heads so presumably 270 MB per platter, almost three times
the capacity of Zip media which came out in 1994. So one might think
Zips used old encoding since they weren't exactly top of the line in
storage density back then.

Well, it looks like Gibson Research confirms it, RLL was used, at
http://www.grc.com/tip/codknowledge.htm which is about the click of
death.
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