After clicking enable usb what happens. Look in your file system to see if its loaded as an external drive.
What O/S are you using.
My OS is Window XP, click my computer there is nothing show Archos or external device, unlike I use to connect my older Archos (405) and external Hard Drive, the Laptop recognize them in no time. It is really weird.
I've just bought my new Archos 605 Wifi from B&H Photo 3 weeks ago. Should be loaded with most recent firmware etc.
Go into the Menu/Settings/System and see what your USB Preference is saying. If it isn't saying PC Hard Drive then switch it and see what happens. If you still need help, go to http://forum.archosfans.com