Ensure that your account has enough permissions to do the operation or that you’re list as a site collection administrator for the site collection that you are restoring.
Your site collection might also be locked by a previously failed restore job : stsadm -o setsitelock -url “http://yoururl” -lock none
(and don’t worry, even when the backup is on the same disk as the sql server, it’s awfully slow to restore… Don’t kill the process and get a long coffee break !)
