I recently bought one of these, mainly because it was in unused condition, and claimed OS9 compatibility.
Hmm, that claim is stretching it a bit.
The device does not present itself as a mass storage device.
You have to use the iRiver Music Manager software (which requires os9.2.2) to communicate with it.
And it's complete junk. Its behaviour is inconsistent, it keeps quitting, most menu items do nothing, and I've failed to transfer any files to it (just get errors or no response).
Anyone managed to use one satisfactorily with their Mac?
I've seen references to updated software and firmware, but not found any working links.