Two things to check on. If you have two HDDs with the same assigned Volume Letter, like C, one won't show up in Windows Explorer, or there will be a conflict. With both disks connected, go to Disk Management and choose the 320 GB drive. Specifically assign it a different drive letter, like "K" for backup.
Second thing is, both will have the "Active" parameter assigned, as both are used as the Active drive. You should be able to see how both HDDs are set up in Disk Management. The 500GB HDD should have 2 Partitions, the first MS Reserved should say Healthy (System, Active, Primary Partition) The Second Partitions should be labeled C:, and say Healthy (Boot, Active, CrashDump, Primary Partition). If the MS Reserved Partition is not on the new 500GB drive, it may have been installed incorrectly Also, make sure in the BIOS boot order, the first HDD to boot from is the 500Gb drive.
It may be changing the drive letter of the 320GB will cause both to show up