Windows 8.1: administrator account has access denied to other partitions

Solution 1:

You may have restricted the permissions on your partitions to keep access only for administrators.

If this is the case, you face a Windows's bug. This bug is since Windows 7 I think. Seems nobody found a clean way to avoid it. Some people sais this is a bug into Explorer, but the command line exhibit the same behavior, as PowerShell, as other applications I tested.
It only fire when UAC is enabled and you belong to the Administrators group and the folder is restricted to the Administrators group only.

If you want to keep UAC, you have to modify the permissions to allow someone else to read the folders you want. Remember: this user must not belong to the Administrators group.
Personaly I create a special user for this usage only.

To modify the permissions, you may have to disable UAC. Once done, you enable it.

Solution 2:

Check first the date and time of the computer. If incorrect, set and reboot.

Then, check the owner account of the other partitions. If incorrect, reset it to its correct value.

I would even re-grant the accounts and group permissions on these partitions, even if they already exist, to refresh them to their correct values.

One of the many articles treating such changes is this link.