Sorry, I missed this thread.
"System checking" in a loop probably means your YP-R0 is bricked.
It probably occurred after your connected your R0 to the PC to update the Rockbox build then disconnected it. It automatically updated the Samsung library and it failed.
This is an infamous bug in any firmware version. It was much discussed on the forums back to the time but some of them like ABI are now down unfortunately.
If you had the original firmware installed (let's say 1.25), you would be in troubles because only the Samsung recovery tool and a special 301 KOhm USB cable could fix it.
But since you seem to have the Modfirm v2.51 dualboot (from Lorenzo92 by the way. VanniX did the Rockbox-only LightROM 4.6), it can be easily fixed with the Safe Mode.
Let me know here if you still need help.