http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF- 8&threadm=3c90ea88%40news.svn.net&rnum=1&prev=/ groups%3Fhl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF- 8%26q%3Dhd20%2Brodime%2Bgroup%253Acomp.sys.apple2%26btnG%3DGoogle%2BSear ch
It contains a pretty good description of what is inside the HD20 and some speculation from knowledgeable people about how it works.
If you find that an HD20 doesn't work, this snippet (from a net.micro.mac usenet posting in the 1980s) about the HD20 test utility may be useful:
"Jeff I tried it and it worked fine. For those curious, if you hit Cmd-D at the
initial dialog box on the HD20 test it then shows two windows: one has boxes
containing Block Count, Iteration, Soft Rate, Hard Rate, Comm Rate, Loops,
Total Blocks, and Failure Code. There is also a radio button labelled Dstrct
which, I believe, will allow you to reformat the hard disk. The program also
creates a document file that has a log of the tests done and the results."
The HD20 test app is on the boot disk originally supplied with the drive. Apple allegedly also had a couple of in-house utilities called HD Diag and Scavenger Mac for fixing HD20s.
I'll summarise the information and post it on a web page when I get the chance.
Phil --- http://www.mandrake.demon.co.uk/Apple/
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