Do you get an error message?

Which device is the vmware hard drive which you are booting from? sda1 sounds like a good guess. Probably your USB stick is on sdb or sdc, or even sdd. Try sudo fdisk -l -- your USB stick will probably have a single partition, and the size should match the size of the stick. Also try look in the system log files for messages pertaining to the insertion of a removable device. (Are you sure it's not being automatically mounted? Many modern Linux distros will do that for you by default.)