This has happened to me twice now. once with an external hard drive and once with an orange 3G dongle. After a bit of research on the net it is caused by the high current drain of these devices. I have tested with a memory stick and it is fine. Also if you plug in prior to switching on it is fine.
A bit more research and it only affects the port on the left side of the asus. If you use one of the dual ports on the right hand side it handles it ok without a power shut off.
Mildly irritating and if you didn’t think to do a search on google you might think you had a faulty netbook not a bad design flaw. Got my details from here.