If your 320hdd is ntfs and you already got all your games on it (i use :\GAMEZ\ as folder on usb) and don't want to format again then you can make multiman reads your ntfs-hdd. But with this mehtod i NEED to copy the games to internall hdd. you canNOT play from external if NTFS.
So if you got enough internal space then edit the USB.CFG (with notepad or similar) to match your VID & PID of your hdd and copy the file on via usb-stick onto your ps3-internal-hdd in this location:
To get into filemanager-mode in multiman just hold select and press start