you should be able to create a backup of your phone on the new hdd. do that first before trying to sync so you don't loose anything.
make sure on iTunes under preferences>devices that automatic syncing is turned off
then right click on the name of your phone under devices in the left hand menu, should be an option to backup phone from there.
from same menu you can also select transfer purchases
then you can try syncing and if it doesn't work at least you have the backup