I get that very message at timeS, to PLUG USB CABLE
My computer "ding dongs" when its plugged in, I will get the ROCKBOX banner on the screen and at the screen bottom it says BOOT 4.0
Normally I would get the USB picture indicating its connected from the computer and recharging...
Now I get a message of USB CABLE or I get a LOADING ROCKBOX message that never loads.
If I can get the player to initialize after turning it off and on many times, then I will have to immediately plug the player into the USB port to recharge or transfer music or delete files etc and afterward it will work perfectly until it shuts off, then the trouble of getting it to initialize again repeats