I get this problem with various usb pen drives on my Intel Mac as well. It seems to be a particular problem with some of the newer Kingston 4GB drives, in particular, those coloured blue that "fold out". Doesn't happen on any other types. Can't mount it on the Mac which replies with a message that the drive is not readable, but it mounts fine on windows systems. You may not have an OS problem but rather its just the usb pen.
Back up the pen data and try reformatting it - it's worked for me.
Also, some pens using a compression utility which starts on insertion, this is windows based software and has the same effect that you have been noticing on the Mac.
Hope that helps