The way I have done this in the past is to put a flag file on the CD or USB media that I am using to store an image - eg xxFLAGxx.TXT and then use a batch file to look for this flag file on each drive letter in sequence until it finds it. eg:
@ECHO OFF
IF EXIST D:\XXFLAGXX.TXT GOTO D
IF EXIST E:\XXFLAGXX.TXT GOTO E
IF EXIST F:\XXFLAGXX.TXT GOTO F
IF EXIST G:\XXFLAGXX.TXT GOTO G
IF EXIST H:\XXFLAGXX.TXT GOTO H
IF EXIST I:\XXFLAGXX.TXT GOTO I
IF EXIST J:\XXFLAGXX.TXT GOTO J
IF EXIST K:\XXFLAGXX.TXT GOTO K
IF EXIST L:\XXFLAGXX.TXT GOTO L
IF EXIST M:\XXFLAGXX.TXT GOTO M
IF EXIST N:\XXFLAGXX.TXT GOTO N
IF EXIST O:\XXFLAGXX.TXT GOTO O
IF EXIST P:\XXFLAGXX.TXT GOTO P
IF EXIST Q:\XXFLAGXX.TXT GOTO Q
IF EXIST R:\XXFLAGXX.TXT GOTO R
IF EXIST S:\XXFLAGXX.TXT GOTO S
IF EXIST T:\XXFLAGXX.TXT GOTO T
IF EXIST U:\XXFLAGXX.TXT GOTO U
IF EXIST V:\XXFLAGXX.TXT GOTO V
IF EXIST W:\XXFLAGXX.TXT GOTO W
IF EXIST X:\XXFLAGXX.TXT GOTO X
IF EXIST Y:\XXFLAGXX.TXT GOTO Y
IF EXIST Z:\XXFLAGXX.TXT GOTO Z
ECHO EXTERNAL DEVICE NOT FOUND - BACKUP PROCESS ABORTED!
GOTO END
:D
D:
CD\
Your code here
GOTO END
:E
E:
CD\
Your code here
GOTO END
:F
F:
CD\
Your code here
GOTO END
:G
G:
CD\
Your code here
GOTO END
:H
H:
CD\
Your code here
GOTO END
:I
I:
CD\
Your code here
GOTO END
:J
J:
CD\
Your code here
GOTO END
:K
K:
CD\
Your code here
GOTO END
:L
L:
CD\
Your code here
GOTO END
:M
M:
CD\
Your code here
GOTO END
:N
N:
CD\
Your code here
GOTO END
:O
O:
CD\
Your code here
GOTO END
:P
P:
CD\
Your code here
GOTO END
:Q
Q:
CD\
Your code here
GOTO END
:R
R:
CD\
Your code here
GOTO END
:S
S:
CD\
Your code here
GOTO END
:T
T:
CD\
Your code here
GOTO END
:U
U:
CD\
Your code here
GOTO END
:V
V:
CD\
Your code here
GOTO END
:W
W:
CD\
Your code here
GOTO END
:X
X:
CD\
Your code here
GOTO END
:Y
Y:
CD\
Your code here
GOTO END
:Z
Z:
CD\
Your code here
GOTO END
:END
ECHO.
ECHO.
ECHO PRESS ANY KEY TO CONTINUE
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
Where I have put "Your code here" - you would put in the appropriate Ghost command to run from that drive letter. Of course you do not need to put in the whole alphabet if you are certain that the target drive letter will always be near the beginning of the alphabet.