1.65 firmware with rockbox bootloader works fine for me.
This is what I did.
- Start Disk Utility
- Select the players drive (XX.X GB Toshiba ML...)
- Select the partition tab
- Change the partition scheme to 1 Partition
- Change the partition name to something nice and makesure the format is Ms-Dos
- Click the partition button and wait.
- Copy files to the player as normal.
I'd recommend using the rockbox bootloader if your going to mess about with the drive. The iriver firmware can refuse to boot if it doesn't like the disk leaving you unable to get into usb mode. The rockbox bootloader will go into usb mode whatever state the drive is in.