I got a Bell Canada Atrix HD from a co-worker. When I used Myth to flash from stock Bell to stock ATT, it would not work properly. When I used RSD with 98.4.20 without going through Myth, the ATT ROM flashed without any errors. I was able to take the OTA update, flash CWM, and revert to a custom ROM.
edit: my bootloader is unlocked, could this have something to do with it?