Windows 7 Computer folder, by default displays your basic hard drives. But if you want to show all the drives, including the empty ones, like say a memory card reader, in your Explorer, you can do so as follows.
Show all Drives in Computer folder
Open Explorer > Folder & Search options > View > Advanced settings > Uncheck Hide empty drives in the Computer folder > Apply > OK.
All drives will now always be visible. If you wish to reverse the changes, simply check the option and click on Apply again
This option is there in Windows 8.1, where you can show all hidden drives too in the This PC folder.