Maybe a little OT but it is important that the 276c is fed by external power when maps are transferred to it. Even with a fully charged battery I have experienced some strange effects, not that I say that this is the problem in this case.
Since the 276c (and 278/478) is not sold anymore and starts to be difficult to get hands on as used I do not want to test with my dear unit. It serves as prime navigator on my Motorcycle and there are still no units that matchs it when it comes to functionallty and robustness.