USB on the Sansa Fuze is buggy under Rockbox. This is a known issue (I use a Fuze daily, and have this issue myself) and they are working on it. Saratoga has the best advice: Boot into the original Sansa firmware to transfer to the player's internal memory.
I use a 64GB micro SD card for external storage, and the original Sansa firmware will not recognise it (I believe it has issues with cards over 8GB). To transfer music to the SD card I pop it out and use a card reader.
These are very minor workarounds, and I feel are worth doing for the advantages Rockbox gives you on this player. If you don't want to use the original firmware or a card reader to transfer files, I think Rockbox version 3.9 worked--but I could be wrong.