Using an external hdd for an xbox and pc?

Buy a router which supports USB mass storage devices, and connect both your PC and your Xbox to it.

or a nas device might give better performance then usb.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

thanks. Those options seem expensive. I was hoping a splitter was possible, but if it isn't I will just connect it to the device I need. So a splitter is out of the question?

You can buy a switch that switches one USB peripheral between two USB hosts. However I would think that switching a USB HDD would be somewhat dangerous. You would need to ensure that you had safely ejected the USB mass storage device before switching it over, in the same way that you would need to safely eject before unplugging the USB cable in a regular single host setup.

OTOH, if you wish to access the drive from both the PC and Xbox at the same time, then this would be even more dangerous. For example, what would happen if the PC were to delete a file that the Xbox was working with, or vice versa? What would happen if two PCs tried to update the same spreadsheet or document? To avoid such problems you would need a separate file server to manage the HDD and to process read/write commands from both hosts. That's what a NAS does, ie it has an embedded file server.