Issue while upgrading firmware

[Ivo Emanuel Gonçalves]

Because during my last auto-upgrade (from Build 635 to 653), the
firmware was not upgraded in the process, I am trying now to do it
manually.

I have ran into a problem, though.  During the OFW "ok" prompt, I type
"flash u:\q2d07.rom", the systems reads the new firmware, but then
gives me the following message: "AC not present" and stops.  What am I
missing here?  What is AC exactly?

For reference, this is a B4 machine, and I'm upgrading from q2d03 to q2d07.

-Ivo