I just flashed the way I did with my DHD..
Extracted the radio image into a folder on the desktop.
Cd'd into the directory from CMD, Rebooted the device into bootloader so;
C:\Users\Dan\Desktop\Radio>adb reboot bootloader
Waited for it to go into the Bootloader..
C:\Users\Dan\Desktop\Radio>fastboot flash radio radio.img
< waiting for device >
sending 'radio' (22141 KB)...
OKAY [ 3.341s]
OKAY [ 2.778s]
finished. total time: 6.124s
finished. total time: 0.988s
Worked fine, Always found this much easier than faffing around with moving stuff onto SD cards - all done from your PC then, You could probs even batch file it if you trusted it enough