I ran into this same problem. I rooted 2.3.3 and, without thinking, took the update without backing up. I tried flashing and got the A5,70,70,00,1F MEM_MAP Blank error as well. To fix I went here: Motorola SBF Files
and downloaded the 621.sbf file for my DX. After that I flashed using the sbf flash method in linux and it fixed my situation. Now if I could only get my root back. Hope this helps.