MSCDEX.EXE AND DOS 6.0
Modify your autoexec file to call the *new* mscdex which should be located in
your dos6 directory, not an older version elsewhere on disk or one that came
with the drive. If that's not the problem, the only other thing I've seen throw
the incorrect dos version error is failing to use setver with dos5, but that
shouldn't be you with dos6.