I have the same problem! I installed rockbox on my iPod classic 80GB, everything works fine, but now I need uninstall rockbox. I connected iPod to PC and enter DFU. But here is problem - iTunes said "An iPod has been detected, but it could not be identified properly. Please disconnect and reconnect the ipod, then try again."
I tried it several times, but nothing changed. I tried all USB ports on my PC, I tried 4 genuine USB cables, I tried reboot PC, I tried Uninstall iTunes and instal it again, but always get the same error. I borrowed my sisters PC and install iTunes and try DFU - again the same problem! (on my PC is windows 7 64b, sister have windows 7 32b)
I tried different iPod 80GB (never rockboxed) and again iTunes said same error! I tryed DFU with next iPod classic 6G 160GB and same error again! Then I tried it with 7G 160GB and again same error!
Only first ipod is with rockbox, other three are with ofiginal firmware (never rockboxed) and DFU is not working!
Very sorry for my english