Go back to Disk Management and look closely in the right spot. It has two main panes on the right. Look at the LOWER RIGHT pane, and realize that it SCROLLS so you can see all it has. It will show you all the hardware storage devices, so if BIOS can detect it, it should be there. Even devices Windows can't understand yet will be there. Each is represented by one rectangular box with a bit of info on its left end. All HDD units should have a smaller box inside with further info about the Partition(s) on that HDD unit. That info includes a name assigned at time of creation, a letter name like "D:" that windows uses for this "drive", a size in GB, a File System like "NTFS", and a Status note.
If your old 320 GB unit has NOTHING on it, it will all be occupied by "Unallocated Space". But you have stuff on it, so there should be at least one Partition block in it. Look closely at the end of the first line of its Partition info for the letter name. If there is none, RIGHT-click on the Partition area and, from the resulting mini-menu, choose a letter for it. When done, back out of Disk Management and reboot so Windows can update its Registry and use the drive.
If instead the File System is shown as "RAW", you have a different problem. If that's what you find, post back here for further advice.
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