Are you saying that once you load a game successfully with multiMAN and then when running the game, once you insert the USB gadget the game freezes? If this is the case, then it's no longer a multiMAN issue, but more like the USB gadget is causing a malfunction on your PS3. Is this the case?
I am hoping you can PM deank and see if there is a way to isolate your USB game drive to be the only device to get scanned in multiMAN. Do let us know what he tells you on how to go about editing the ini file--so we all can learn from his examples. Good luck!