@DISKUSED[d:[,scale[c]]] : Returns the amount of disk space in use on the specified drive. If you're specifying a drive, the drive letter must be followed by a colon. Optionally, you can specify a directory or UNC name, and @DISKUSED will display the disk space in use on the drive referenced by that name (which may be different from the drive if the directory is a link to a directory on another drive).
The optional second parameter specifies the reporting scale (see Memory Size / Disk Space / File Size Units and Report Format). If the scale specification is suffixed with c the result will be formatted using the thousands separator.
@DISKUSED supports OpenAFS names.
See also: @DISKFREE and @DISKTOTAL.
Examples:
echo %@diskused[c:]
100090298368
echo %@diskused[%_disk:,Kc]
472,947,772