Option Explicit
Dim i, aAvailableDriveLetters
For i = 65 To 90
If CreateObject("Scripting.FileSystemObject").DriveExists(Chr(i) & ":") Then aAvailableDriveLetters = aAvailableDriveLetters & VbCrLf & Chr(i) & ":"
Next
MsgBox aAvailableDriveLetters