The problem could be a lot of things. USB driver for instance, or as bob suggested, filesystem driver, although that seems unlikely if it happens with fat as well.
You could if you have the possibility try with ext4. It's one of the most stable filesystems on Linux. Mac OS X however doesn't support ext filesystems, but there are some projects that give you read only support of ext4 on OS X.
You may also want to create a log with MC. Go to Help/Logging and enable Output to a log file. If it is already enabled, reset the log at the bottom. Reproduce the problem, go back to Help/Logging and click on Report Problem. Attach the log to a forum post. Maybe someone at jriver can interpret the log and see what the problem is.