I've seen some other postings about higher addresses failing, but none about block 0 failing.

This is a "27 menu" model of the uv-b5.

Downloading works fine.

Unmodified image here: http://vcr.einic.org/~nyx/uv-b5_orig.img

I've tried uploading both modified and completely unmodified images to the radio. All fail with the same "Radio NAK'd block at address 0x0000" error.