Are these drive letters assigned to different hard drives or are they just logical partitions on one hard drive?
Make right click on the one drive letter and click properties. Check the security tab to verify that you have access to that drive. Sometimes even if you are logged as an administrator, you could lose rights to the location. Check the security tab to see who has rights on that drive letter.
I found also some tips on this forum that you might want to check: https://ccm.net/forum/affich-458737-i-can-t-open-my-local-disk-drive-e#2