5.25" drive presence (was Re: Undeleting HFS subdirectory?)

[david@wraith.cs.uow.edu.au (David E A Wilson)]

dempson@swell.actrix.gen.nz (David Empson) writes:
>Under System 6.0 and earlier, the Apple 5.25" driver polls the second
>drive during boot to see if it is there.  The usual method for testing
>for a drive's presence is to turn on the motor and watch the incoming
>data line.  If it is changing, the drive must be present.  If it
>doesn't change, there is probably no drive.

>Some third-party 5.25" drives provide a constant data signal if 
>there is no disk in the drive, so this method mistakenly thinks that
>the drive isn't present.

This sounds like the trouble I have with one of my 5.25" drives on my //e.
One of the drives cannot be used to format a blank disk on the UDC controller
or using FILER on a Disk ][ controller. The other two drives have work fine.
It turns out that one drive reads a blank disk as all ones while the other
reads it as all zeros.
-- 
David Wilson				+61 42 213802 voice, +61 42 213262 fax
Dept Comp Sci, Uni of Wollongong	david@cs.uow.edu.au