ASSIGN [d1=d2 [...]] 

The ASSIGN external command tells DOS to use another disk drive than the one 
specified. The ASSIGN command can switch these names. 

    ASSIGN [d1=d2 [...]]

d1 and d2 are disk drive letters (without the colon (:)). After an

    ASSIGN A=C

command, all references to drive A: are routed to drive C:. It's as if A: 
doesn't exist. SUBST C: A:\ is a more flexible way to do this. An ASSIGN 
command with no options will undo the effects of all previous ASSIGNs. 

Any ASSIGN commands used must appear after the first APPEND command. Trying 
to use BACKUP, CHKDSK, DISKCOMP, DISKCOPY, FDISK, FORMAT, JOIN, LABEL, REPLACE,
RESTORE, or SUBST on a disk drive letter defined by ASSIGN should be avoided.

CAUTION: COPYing from A: to C: with this ASSIGN is hazardous. If the filename 
is not changed by the COPY command, a file can be totally or partially lost. 
