I don't own the device but I wouldn't even attempt a Sandisk firmware on it (let alone Rockbox), you should just try to get it back to its own firmware.
Since you have disk mode available would suggest looking for an official firmware update for the device. If it goes into the root of the disk you might be able to get away with dropping those files there, rebooting the device and it might get you back to where you started.
Again I don't own this device but certain players like the Cowon D2 are like this where you can get back to a working firmware, as long as you have disk mode access.