If it's an older card I would try that. i am certain at this point mine was hanging due to my 2 year old card being corrupt/damaged. Tried filling it and windows only copied 10gb before failing on a 16gb card even after low level format.
Its worth a try. UPS should be here in an hour.... counting the seconds S400 and 64gb on that truck!
It is a 3 month old Lexar 32GB high speed card, and I'm pretty sure it's not the problem. Regardless, I reformatted it and copied back all of my tunes. Same problem with rockbox on my clip zip. Occasionally a song will be playing fine, and then it just seems to pause without any recourse except to reboot. It has hung on mainly mp3 files (various bit rates), but I'm pretty sure that it has also happened with aac files that I have downloaded or ripped via iTunes. And what makes me rather sure that it's rockbox and not the card is that when I boot into the Sansa firmware and play the songs there, the problem does not occur.
Although the Sansa firmware is fine (and a great backup), it definitely is lacking a lot of features that I like in rockbox, so I'm going to have to get googling!