I have 2 consoles and use 1 HDD (old style) between the two.
When i update my home console and then take my HDD to work, my work console updates, then back home and requires update again. Constantly both consoles need the update, this has never happened before, usually i take my work console home and update it on my broadband. I recently hooked up a dongle though and use that to connect to LIVE whilst in work so my achievements get time-stamped.
I think i'm going to try taking my console home on the weekend, but if i can get it fixed today with a better solution i will do that.