I am working on a project that needs the Arduino to only work if the power supply is connected and not if only USB is connected. So a quick search told me to remove the polyfuse from the board which I did without researching any further.

When I tried to connect the Arduino to the pc it wouldn't connect and a further search (which I should have done before going ahead and desoldering the fuse) told me that the fuse is needed to connect.

A little prodding about on the board told me that all was not lost (removing the fuse had destroyed it) and if I supplied the 5V the pc gave a pleasant noise that said "there it is".

Luckily there is 5v really close to where the now deceased fuse used to be (the regulator) so a simple bit of wire solved the issue and now its working exactly the way I wanted it to with the possible exception of I may have wanted to upload new code without the power supply attached but this is an acceptable work around.

Please excuse the ropey soldering but the arduino is in place and would have took a lot of work to get it out to make a neat job.