This restriction prevents users from using My Computer or Explorer to access the content of selected drives. Also, they cannot use Run, Map Network Drive, or the Dir command to view the directories on these drives.

Open your registry and find or create the key below.
The "NoViewOnDrive" value uses a 32-bit bitmask to define local and network drive access for each logical drive in the computer. The lower 26 bits of the 32-bit word correspond to drive letters A through Z. Drives are visible when set to 0 and hidden when set to 1.

If your not happy working in Hex, add these decimal numbers to hide the drive(s):

A: 1, B: 2, C: 4, D: 8, E: 16, F: 32, G: 64, H: 128, I: 256, J: 512, K: 1024, L: 2048, M: 4096, N: 8192, O: 16384, P: 32768, Q: 65536, R: 131072, S: 262144, T: 524288, U: 1048576, V: 2097152, W: 4194304, X: 8388608, Y: 16777216, Z: 33554432, ALL: 67108863

For example to hide drive A and drive D, you would add 1 (A) + 8 (D) which means the value should be set to "9".

To disable all the drives set the value to "67108863".

Restart Windows for the change to take effect.

Registry Settings
User Key: [HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\
Explorer]
System Key: [HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\
Explorer]
Value Name: NoViewOnDrive
Data Type: REG_DWORD (DWORD Value)
Value Data: 32-bit bitmask