I was having a weird problem with my 512 Jasper last night. I could dump part of the NAND then all the sudden I would get read errors in the 2BXX area. It wasn't the same each time, but always started around 2B. After I would get the read errors, it would give me the looking for USB, then cannot detect error. I then would have to reset nandx pull the power on the xbox, and after a few runs at this it would detect again.
I started with qsb, the moved to pin headers, both same results. Eventually I pulled out the pin headers, to look at the traces, everything looked good. Not wanting to resolved the pin headers, I direct wired it. Still same issue. I saw another post from someone else about using a different psu. I tried it with an old xenon (203w) psu and to my surprise it worked just fine. Maybe my ground was just barely making a connection, but it worked for me so I didn't keep messing with it.