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The NoDrives value can be used to selectively hide drives because the rightmost bit of this hexadecimal value represents the A: drive, and the leftmost bit represents the Z: drive. To hide a specific drive, turn on its bit. Use the following decimal numbers to hide the given drive:

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.

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This policy displays only specified drives on the client computer. The registry key that this policy affects uses a decimal number that corresponds to a 26-bit binary string, with each bit representing a drive letter:

11111111111111111111111111
ZYXWVUTSRQPONMLKJIHGFEDCBA
				
This configuration corresponds to 67108863 in decimal and hides all drives. If you want to hide drive C, make the third-lowest bit a 1, and then convert the binary string to decimal. 

It is not necessary to create an option to show all drives, because clearing the check box deletes the "NoDrives" entry entirely, and all drives are automatically shown. 

If you want to configure this policy to show a different combination of drives, create the appropriate binary string, convert to decimal, and add a new entry to the ITEMLIST section with a corresponding [strings] entry. For example, to hide drives L, M, N, and O, create the following string

00000000000111100000000000
ZYXWVUTSRQPONMLKJIHGFEDCBA
				
and convert to decimal. This binary string converts to 30720 in decimal. Add this line to the [strings] section in the System.adm file: LMNO_Only="Restrict L, M, N and O drives only"

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// Managing Register services from FiveWin

#ifndef __XPP__
   #define  HKEY_CLASSES_ROOT       2147483648
   #define  HKEY_CURRENT_USER       2147483649
   #define  HKEY_LOCAL_MACHINE      2147483650
   #define  HKEY_USERS              2147483651
   #define  HKEY_PERFORMANCE_DATA   2147483652
   #define  HKEY_CURRENT_CONFIG     2147483653
   #define  HKEY_DYN_DATA           2147483654
#else
   #define  HKEY_CLASSES_ROOT       1
   #define  HKEY_CURRENT_USER       2
   #define  HKEY_LOCAL_MACHINE      3
   #define  HKEY_USERS              4
   #define  HKEY_PERFORMANCE_DATA   5
   #define  HKEY_CURRENT_CONFIG     6
   #define  HKEY_DYN_DATA           7
#endif

//---------------------------------------------------------------------------//

function Main()

   local nHandle, cValue

   if RegOpenKey( HKEY_CURRENT_USER ,;
         "Software\Microsoft\Windows\CurrentVersion\Policies\Explorer", @nHandle ) == 0

      RegQueryValue( nHandle, "NoDrives", @cValue )
      MsgInfo( cValue )
      RegCloseKey( nHandle )
   else
      MsgAlert( "Can't open this key" )
   endif

return nil

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how to hide drive 'C' ( which is very foolish )

ZYXWVUTSRQPONMLKJIHGFEDCBA ( all drive letters )
00000000000000000000000100 ( binary settings )
you'll have to put '4'     ( decimal value of above binary )


Better for testing try to hide drive 'D'

ZYXWVUTSRQPONMLKJIHGFEDCBA ( all drive letters )
00000000000000000000001000 ( binary setting )
you'll have to put '8'     ( decimal value of above binary )

And here an example for hiding drives 'E' and 'D'

ZYXWVUTSRQPONMLKJIHGFEDCBA ( all drive letters )
00000000000000000000001100 ( binary setting )
you'll have to put '12'    ( decimal value of above binary )

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Function Dec2x(pnNo, pnBase)

  Local cRetVal              := ""
  Local nCount               := 0
  Local acDigits             := {"0", "1", "2", "3", "4", "5", "6", "7", "8", "9"}
  Local nNo                  := 0

  For nCount := 65 to 65 + pnBase - 11
    aadd(acDigits, chr(nCount))
  Next

  For nCount := 7 to 0 step - 1
    nNo                      := int(pnNo / (pnBase**nCount))
    If nNo > 0 .or. !empty(cRetVal)
      cRetVal                += acDigits[nNo + 1]

      pnNo                   -= nNo * pnBase**nCount
    Endif
  Next

Return cRetval

//------------------------------------------------------------------------------------------------------//
Function x2dec(pcNo, pnBase, plNoWarningMsg)

  Local nRetVal              := 0
  Local nPos                 := 0
  Local acDigits             := {"0", "1", "2", "3", "4", "5", "6", "7", "8", "9"}
  Local nCount               := 0
  Local nStop                := 0
  Local cDigit               := ""

  DEFAULT plNoWarningMsg     TO .t.

  For nCount := 65 to 65 + pnBase - 11
    aadd(acDigits, chr(nCount))
  Next

  nStop                      := len(pcNo)
  For nCount := 1 to nStop
    cDigit                   := substr(pcNo, nStop - nCount + 1, 1)

    nPos                     := ascan(acDigits, cDigit)

    If nPos > 0
      nRetval                += (nPos - 1) * (pnBase ** (nCount - 1))

    Elseif !plNoWarningMsg
      // pk 20/9/2002 msginfo("Invalid number " + pcNo + " " + alias() + " " + NTRIM(recno()))
      msginfo("Warning - function x2dec(), invalid parameter value " + pcNo, "Warning")

    Endif

  Next

  nRetval                    := int(nRetval)

Return nRetval

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#define HKEY_CURRENT_USER   -2147483647  // 2147483649

oReg := TReg32():New(HKEY_CURRENT_USER, "Software\Microsoft\Windows\CurrentVersion\Policies\explorer")

oReg:Set("NoDrives", ltrim(str((nNumber)))

oReg:End()

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PROCEDURE Main()
   cls

   ? nDriveNum( "EDA" ) // getting number to hide drives E, D and A
   ?
RETURN

/////////////////////////////
FUNCTION nDriveNum( cDrives )
/////////////////////////////
LOCAL nRet := 0
LOCAL n    := 0



   for n := 1 to len( cDrives )
      nRet += 2 ^( asc( upper( substr( cDrives, n, 1 ) ) ) - 65 )
   next

RETURN( int( nRet ) )

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#INCLUDE "fivewin.ch"

  #define HKEY_CURRENT_USER   -2147483647  // 2147483649

PROCEDURE Main() 
   cls 

   ? nDriveNum( "C" ) // getting number to hide drives E, D and A
   ?
   registra(nDriveNum( "C" ))
RETURN

/////////////////////////////
FUNCTION nDriveNum( cDrives )
/////////////////////////////
LOCAL nRet := 0
LOCAL n    := 0



   for n := 1 to len( cDrives )
      nRet += 2 ^( asc( upper( substr( cDrives, n, 1 ) ) ) - 65 )
   next

   RETURN( int( nRet ) )
FUNCTION registra(nNumber)


oReg := TReg32():New(HKEY_CURRENT_USER, "Software\Microsoft\Windows\CurrentVersion\Policies\explorer")

oReg:Set("NoDrives", nNumber)

oReg:Close()

RETURN nil