Usb not recognized problem when connected to mains voltage?????

[humbug0_2]

Hi there
I wonder if anyone can help.
I am basically having some strange problems with my laptop recognizing my external hard drive.
If i plug the usb into my laptop without the laptop power lead plugged in all works fine.
But as soon as i plugged the lead in to charge the laptop the external hard drive seem to lose connection with the laptop and i get the message about usb not recognized,malfunction etc.
Anyone know why my laptop charger is doing this?
I have plugged this into my desktop pc and all works fine.
I'm stumped!