Available drive letters

My colleague needed to run subst command and therefore was looking for available drive letters. During the script creating I found out that running simple InStr() function is slower then to use Dictionary object (maybe it's a tip for other blogpost). Anyway I used InStr() because I stopped the script after first match.

Const wbemFlagReturnImmediately = &h10

Const wbemFlagForwardOnly = &h20

Set WshNetwork = WScript.CreateObject("WScript.Network")
Computer = WshNetwork.ComputerName

Set objWMIService = GetObject("winmgmts:\\" & Computer & "\root\CIMV2")

Set colItems = objWMIService.ExecQuery("SELECT * FROM Win32_LogicalDisk", "WQL", wbemFlagReturnImmediately + wbemFlagForwardOnly)

For Each objItem In colItems

Disks = Disks & lcase(left(objItem.DeviceID,1))

Next

For I = 122 To 97 Step -1

Character = Chr(i)

If InStr (Disks, Character) Then

Else

Result = Character

Exit For

End If

Next

WScript.Echo "Available drive letter: " & Result

The script connects to WMI and query all available logical disks. Then is stores the drive letter in string named Disks. As the drive letter is stored in DeviceID in form C: I converted it to lowercase and remove the colon. After the whole WMI part the Disks looks like this: "cdehpvx".

Then in For loop I am going thru alphabet from the end to the beginning (to say it exactly - from 'z' to 'i') and after first non-match exit the loop and return the letter.