physical disk access

Bob McGowan
Thu Dec 30 13:28:00 GMT 1999

I am doing disk testing of physical devices on NT.  Under NT, I can
access the physical disks using names of the form "\\.\..." where the
... can be a drive letter (including the colon) or a name like
"physicaldrive2".  The physical drive scheme numbers the drives starting
at 0, in sequence for all drives found.

I wanted to use dd to verify disk sizes on reads and writes, so I used
the following two commands:

  dd if='\\.\F:' of=/dev/null bs=1024k
  zero | dd of='\\.\F:' bs=1024k

where the command zero just generates null bytes on standard out.

The first command runs fine and reports the expected size (300 MB).  The
second fails with "dd:  \\.\F:: Invalid argument".  I wrote a quick
program in C to check error conditions/permissions, using
open("\\\\.\\F:", O_RDWR).  The program can write to this device without
any problems.

I would have expected dd to work for both in and out operations under
these circumstances.  Any idea why it didn't?

I am running the Cygwin 1.0 environment from the CD, no updates.

Bob McGowan
Staff Software Quality Engineer
VERITAS Software

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