On W7 there seems to be a problem with PSUBST. After using it to assign a:, the following message appeared then I tried to unassign it
:\howard\working>psubst/d a:
TCC: (Sys) The specified program requires a newer version of Windows.
"SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices"
Also PSUBST with no arguments gives the /= screen both when there is and is not any substitutions.
The help description for PSUBST says PRIORITY rather than PSUBST in the documentation for the /= opetion.
:\howard\working>psubst/d a:
TCC: (Sys) The specified program requires a newer version of Windows.
"SYSTEM\CurrentControlSet\Control\Session Manager\DOS Devices"
Also PSUBST with no arguments gives the /= screen both when there is and is not any substitutions.
The help description for PSUBST says PRIORITY rather than PSUBST in the documentation for the /= opetion.