I found my own solution, so will post it here in case anyone is listening
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Go to
http://www.iriver.com/support/down_list.asp and choose your model range under "Select a product", and "Firmware" under "Select a category". Don't type anything in the third box. Click "Go" (to the right of the third box). Click on the version of the firmware you want (they call UMS "MSC" for some reason). Then you should see a list of file names with different parts of the world (US, EU, JP etc) in brackets. Click on the one for your part of the world and it should download. Then format your player and upgrade firmware, as per Music Manager documentation. This installs nice UMS firmware which makes the player appear as a USB hard drive under both Linux and Windows (and no "File Error"s so far, after a couple of transfers).
David