Ubuntu Multimedia :: No Space Left On Device After Upgrade To 9.10

I've just upgraded to kubuntu 9.10, and I'm getting strange errors when trying to copy files onto my MP3 player, a SANSA M250 (2GB). df claims it's at 57% capacity (and the figures make sense for that, as well as having about the quantity of music which half fills the SANSA) but when I try to copy a file - from the command line, from exaile, or with the file manager, I get a "No space left on device" error:

Code:
$ df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda1 73939452 42184580 27998880 61% /
udev 512472 264 512208 1% /dev
none 512472 1752 510720 1% /dev/shm
none 512472 96 512376 1% /var/run
none 512472 0 512472 0% /var/lock
none 512472 0 512472 0% /lib/init/rw
/dev/sdc 1996000 1125056 870944 57% /media/SANSA M250
$ cp test.mp3 /media/SANSA M250/
CP: cannot create regular file `/media/SANSA M250/test.mp3': No space left on device
The MP3 being copied is 2MB in size so should fit easily.

There were no problems until upgrading, when I also moved from amarok to exaile - because amarok2 doesn't have the functionality I want from a player. But this happens even if exaile isn't running, so exaile shouldn't be the problem. How can I diagnose what's going on?