if your laptop has USB ports next to each other,
and one of them is USB 3.0,
and the other has a wireless mouse's receiver in it that's 2.4ghz,
then the wireless mouse might not work when the usb 3.0 port is used (like when hooked up to an external hard drive).
read here to understand why: http://www.usb.org/developers/whitepapers/327216.pdf
this happened to me. on my new computer. sucks.
i recently bought a new laptop, a samsung np700z3c. i describe why i bought it here.
it's a beautiful machine, a great reason why apple would want to sue samsung, and it's only 2/3rds the price of a mac book pro. it looks good on the outside and is fast on the inside.
but it has one major flaw - it has 2 usb ports right next to each other, and that usb 3.0 port on the left causes a 2.4ghz radio frequency interference on the wireless mouse receiver that's plugged into the port beside it, screwing over my mouse. (most wireless mouse use 2.4ghz)
samsung laptop designers should've put these usb ports on different sides of the computer.
i believe macbooks have this problem too though, as described here. hehehe, suckers.
interestingly enough, there is a company that makes 5.8ghz wireless mice