If you delete files on your computer, the Rockbox O/S doesn't know that. If you delete them using rockbox it will know. The bitch is that it will shutdown based on the fact that it THINKS the disk is full, when it may not be.
There is a way to refresh it, so it actually looks at the hard drive and updates Rockbox with the right answer. It's a little tricky... sometimes it takes a couple of tries...
- Go into the system menu, Rockbox INFO, where you see the amount free
- push in the joystick button and hold it
- push the PLAY button (top one on the right) for a moment and release it.
- you will see "SCANNING DISK" appear
- you can release the joystick button now.
At least that's the procedure on mine... I suppose it's possible that different versions of Rockbox are different.