Hi 9289,
Since the pictures are still on your internal harddrive, you can try reformatting external harddrive and check.
You may try connecting the external drive to another PC to check if the device is working there. If so take a backup of the data and try formatting the drive.
Note: Before reformatting the drive take a backup of the data.
For more information, you may refer the following links:
http://windows.microsoft.com/en-US/windows-vista/Formatting-disks-and-drives
Hope this information is helpful.