From the iriver FAQ:
[530] Why is Rockbox saying my drive is full when it really isn't?
This has happened to a bunch of people. You delete a bunch of files in windows, but you're getting this damn message in Rockbox saying your disk is full. The reason for this was best described by Petur in another thread:
'The problem you're seeing is that when deleting files through the usb connection (windows/linux/mac/...), rockbox doesn't know about the freed space.
For speed reasons the disk isn't scanned on bootup, and rockbox thinks the clusters are still occupied. We're still looking for a way to get round this, maybe do a scan in the background or set a flag somewhere when we suspect a scan is needed.
To make rockbox rescan the drive:
go to Menu -> Info ->Rockbox Info and tap the play button (or click the joystick).
You should see a Scanning disk message.'
To play it safe you should probably do this every time you delete a bunch of files via windows. This problem does not occur if you delete via Rockbox. To delete via Rockbox, via the file browse just select the file or directory you wish to delete, then click and hold the joystick. A menu will appear. Select delete, and click the joystick another couple of times to confirm. This can come in very handy when you're on the road and realise you don't have enough disk space to record the gig you've just set up for. Smiley