Hmm, now we're getting somewhere. But this sounds bad, really bad. If both cable and your PC is OK and you know this for sure, then the tablet might have something screwed in flash (probably config partition holding the board id) or the USB chip is broken. having such device I would be happy if it still boots and did the boot from mmc hack as soon as possible or your first reboot loop might be your last. No flashing means no recovery. It is vital to have one perfectly booting system (either in flash or mmc or both) so you can fix things in flash directly on the device.

But still it may be problem with cable or that specific PC and its USB interface. Trying other PC or operating system or cable is your only hope.

Does the USB in your N770 work fine otherwise i.e. can you see your MMC card as a storage drive when you connect it to PC when N770 system is booted?