Hello macqs welcome to the forums

There a a few other things to try, although it does seem odd that your PCs are not seeing the n900.

Have you tried installing Nokias software suite from their site? This should come with the software to recognise devices which may help.

Failing that, many prefer to install ssh from the repositories and then install Winscp on to your PC and have full root access to all of the n900s files and folders via wifi connection.