5.25 floppies that read but don't write

Fred Cisin cisin at xenosoft.com
Wed Aug 19 23:10:26 CDT 2015

Just in case anybody was unaware, . . .
VERIFY is NOT a suitable test in this situation.

Most beginners assume that VERIFY will re-read the track, and determine 
whether the write wrote the correct content.  IT DOES NOT.
VERIFY checks the sector headers and CRCs, and determines whether there is 
a valid sector or track there, NOT whether the content within the track is 
what you wanted.

In what this particular case appears to be, if the WRITE cricuitry in the 
drive fails, the computer may state that it wrote to the drive without 
error, and that VERIFY was successful!  But, if the WRITE circuitry fails,
the write may fail, without producing any errors (FDC WRITE error codes 
include seeing WRITE-PROTECT, and a few having to do with not finding the 
desired sector).  Then, VERIFY looks at the same-old content, and declares 
that Yes, the sector or track is a valid one.
What was on the disk before, is what is still on it.

We had this with a couple of TRS80 drives.

We also had some fun when the purchasing department gave us HD disks! 
(for use in TRS80 model 1 SD)
They would sometimes FORMAT, WRITE, READ, and then minutes later, be blank 
The purchasing agent was in bed with somebody peddling RoyType, and kept 
substituting those for whatever we ordered.

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