主要感觉还缺少错误处理的代码，以后加强，（其中利用了动态数组：））

Dim WshNetwork
Dim strComputer
Dim strDrive
Dim objWMIService
Dim colDisks
Dim objDictionary
Dim arrTestArray()
intSize = 0

Set WshNetwork = WScript.CreateObject("WScript.Network")
Set objDictionary = CreateObject("Scripting.Dictionary")

strComputer = "."

Set objWMIService = GetObject("winmgmts:\\" & strComputer & "\root\cimv2")
Set colDisks = objWMIService.ExecQuery("Select * from Win32_LogicalDisk")

For Each objDisk in colDisks

 objDictionary.Add objDisk.DeviceID, objDisk.DeviceID

Next
 
 For i = 67 to 90

    strDrive = Chr(i) & ":"

If objDictionary.Exists(strDrive) Then

Else
    ReDim Preserve arrTestArray(intSize)
    arrTestArray(intSize) = strDrive
    intSize = intSize + 1
  
End If
  
 Next
    
    Wscript.Echo arrTestArray(0) & " is the next available drive letter."
    WshNetwork.MapNetworkDrive arrTestArray(0), "\\macs-szmail1\Data\Pmc"