@DISKFREE[d:[,scale[c]]] : Returns the amount of free disk space 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 @DISKFREE will display the free disk space 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.
@DISKFREE supports OpenAFS names.
See also: @DISKTOTAL and @DISKUSED.
Examples:
echo %@diskfree[c:]
19941240832
echo %@diskfree[%_disk:,Kc]
503,709,632