If it's a new drive then it means that you should format it and assign drive letters. Please go to Control Panel > System and Security > Administration Tools > Computer Management > Disk Management.
If your drive is in good shape and functionality, you will see it in the list as a 320 GB drive with all the space left empty. Right click it and select New Partition and follow the wizard to set up a new partition. A drive letter will then be assigned and the formatted partition will be available for browsing in My Computer.