I recently built myself a new computer that's running Windows 7 64-bit edition. Most things worked fine with it -- until I plugged in my Western Digital Passport portable hard drive. I knew the drive was okay because it was working fine on my other Windows XP 32-bit machine. I finally figured out how to get it working. I'm not sure why, but it turns out that Windows wasn't assigning it a drive letter. To fix the problem I followed these step:
1) Plug in the drive
2) Open the start menu
3) Right click on "Computer"
4) Select "Manage"
5) Under "Storage" click on "Disk Management"
6) Find the external hard drive in the list at the bottom
7) Right click on it and select "Change Drive Letter and Path"
8) Click the Add button
9) Pick a drive letter and okay your way out