This device uses a proprietary filesystem and does not mount like a traditional USB device. You must install drivers which can be obtained here:
http://ifp-driver.sourceforge.net/
...Or see below about a firmware update.

Notes:
ifp-line-0.2.4.6 works fine, if a little slow. When using Midnight Commander to copy files over to the iFP-780 remember to deselect "preserve attributes" or the transfer will fail.

iriverfs-r0.1.0.1.patch.gz worked fine once line 138 of fs/iriver/inode.c was changed from ".memory-backed = 1," to ".capabilities = 0,". Alternatively change line 4296 of the patch before applying it. This is necessary only on recent kernels.

It may be possible to upgrade the firmware so the device supports UMS and this is all unnecessary. To download the UMS firmware upgrade go here:
http://www.iriver.com/html/support/download/sudw_list.asp?searchProductIdx=35

Since my last review I've had the opportunity to test it out some more.

Concerning the UMS firmware, I would recommend that you don't use it - the device does not actually act as a UMS device, the firmware simply emulates a UMS device. This is considerably slower than the other methods. For one thing, after umounting it the device can take upwards of 10 minutes before it is actually detachable.

The method I've had most success with is the ifp-line, especially the MC plugin.