I have a similar problem when plugging in my mobile (Sony Ericsson k750i). It has a memory stick inside which gets handled as an USB device. Everything works perfectly except when running Rhyhtmbox: In that case I cannot unmount the device. So I guess, at least in my case, it is a Rhythmbox problem. I have no idea how to prevent Rhythmbox from blocking the unmount of the device ("Portable Devices/iPod"-Plugin is disabled).
I have a similar problem when plugging in my mobile (Sony Ericsson k750i). It has a memory stick inside which gets handled as an USB device. Everything works perfectly except when running Rhyhtmbox: In that case I cannot unmount the device. So I guess, at least in my case, it is a Rhythmbox problem. I have no idea how to prevent Rhythmbox from blocking the unmount of the device ("Portable Devices/ iPod"-Plugin is disabled).