I have 2 drives (D& E) which are HP system recovery uses only.
Old_drives

To hide drives:
1) Browse to  HCU\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
2) Create a REG_DWORD called NoDrives
3) As I need to hide Drive D& E so my Hexdecimal value is 18*
4) End Process of “Explorer” using Windows Task Manager
5) Open “Explorer” process from Run

Result:
New_drives
Hidden drives are still accessable, just type D: in Address bar.

*18 = 11000 (i want to hide D & E, so set as 1)
Reference: http://www.howtogeek.com/howto/windows-vista/hide-drives-from-your-computer-in-windows-vista/
binary

Below is Binary to Hex Converter WebsiteBin-Hex-Con

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