There are some firmwares that expect the rkusb.tag to be called something else, so all you need to do is rename rkusb.tag to:-
teclast.sys (this is used for Teclast firmwares)
or FRK.TAG this is another I discovered for an unbranded firmware on the net.
So if you find rkusb.tag doesn't work for you try renaming it to either FRK.TAG or teclast.sys and then copy the file on to the player's main memory and try again.
Afaik this
only works for Windows Xp systems and Linux kernel v2.6.27 unless someone can confirm otherwise.
I know some user's have had drive mounting problems with Windows
Vista.
update:-It works on Linux too, I've tried it out, but you need new Linux kernel v2.6.27 and you still need to copy RKusb.tag on to your player.