I have encountered similar problems when attempting to use flasher-2.0 when (1) modifying kernel for higher MMC bus speeds, and (2) when trying to add 2GB rs-mmc support.

In following similar instructions involving flashing zImage-su-18-200625 or 200627, I get the following message after powering up my 770 after the "Suitable USB device not found, waiting" message:

"USB device found found at bus 001, device address 004
"Error claiming USB interface: Operation not permitted"

Does anyone have any ideas on what is happening?

You may need to be root. or try sudo flasher.......

Many thanks. That indeed was the problem.