I've moved it to the USB ports on the back and it seems to be OK, but half of the transfers I do fail !!!

It shouldn't do that... Now that you've got the USB mostly working, please make sure to upgrade to the latest Rio Music Manager as well as the latest Karma firmware. See if that takes care of it. If you continue to have trouble, start a new thread specifically on the topic and we'll work through it to find a solution.