I had this running shortly, until the chain got so big my drive couldn't update because it ran out of space after being off the network for 5 weeks. Now I can't be bothered & just use paper wallets & bitchain when I want to buy something.

You need to modify your shortcut to point to "bitcoin-qt.exe -datadir="     replace with the drive letter and folder your chain file & wallet reside on your flash.

here is a page of other command line options in case you need them
https://en.bitcoin.it/wiki/Running_Bitcoin

Good luck, hope your drive is large enough to make it worth while for you.