This could be an issue where the part of the hard drive you are trying to access is corrupt and not accessible which would explain the reason that it would let you access the other partition.
Now I have seen cases where Malware can break the link of a Hard Drive.
So let's try a couple of steps.
Let's go back in to "Disk Management"
Lets right click on the partition that you are not able to access and select "Change Drive Letter and Paths"
Now try selecting a new Drive Letter.
This will restore the path if it has been broken.
There are also Data Recovery programs that might be very helpful to recover your important information.
Downloads to Recover Your Files and Save Your Bacon and 10 Best Free Hard Drive Utilities.
Some addition links found on Googlehttp://www.datarecoveryreview.net/
http://www.easeus.com/resource/hard-disk-data-recovery.htm
hope this helps
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