An external hard disk would require drivers, since the dash is not really made for that purpose you would require an OS like XBLinux. Regretable that XBLinux is designed to mainly run as an OS and not a gaming system.
In a nutshell, you coldnt use it as large savegame medium (External HDD) however you could use a pen drive but the XB would most likely format it making it unreadable to a PC except via FTP.
