I went to plugin two clips on the same USB hub (apparently unpowered at the time) and one suddenly got dismounted with the usual warning on Mac OS X. From then on the player would freeze when booting at the title screen. I tried removing all of the files and I tried using the firmware update .bin file found on this forum to fix it. The only way to fix was to reformat the player's disk which I did on a Windows machine but imagine you could also do on a Mac machine if you reformat it for FAT32. I'm pretty sure the "MS-DOS File System" choice in Disk Utility for Erase produces FAT32.
I've had no trouble that I can think of just using it as a USB drive to copy and delete mp3 files on the Mac using latest Tiger.