If you can see them in your BIOS, just assign them a letter in the hard drive by doign the following:
1. Go to System and Security in the Control Panel.
2. Select Administrative Tools and then Computer Management.
3. Select Disk Management on the left.
4. Right click on the hidden drive and select Change Drive Letter And Paths.
5. Click Add then click to Assign Drive Letter.
If the missing drives are empty, open My Computer and go to Organize, Folder and Search Options, click on the View tab. Uncheck the box called "Hide empty drives in the Computer folder".