Problem with fastboot flashing

I have somehow installed the fastboot drivers for my Pixel C (although ADB still doesn't work), and have successfully (?) unlocked the bootloader.

If I try to flash either the newest bootloader in order to install the N Developer Preview or a TWRP image, I get the following message:

Quote:

FAILED (command write failed (No such device or address))

I tried different USB ports but the result was the same. I found somewhere about fwtool, but in order to use that I have to be rooted (if I understood correctly).

Any other ideas?