I saw the Get-NextFreeDrive function in this answer and I wondered if there was a more efficient way to do this. It appears that the function in the linked answ
Here's what I came up with. I need the last available drive letter from A to Z.
$AllLetters = 65..90 | ForEach-Object {[char]$_ + ":"}
$UsedLetters = get-wmiobject win32_logicaldisk | select -expand deviceid
$FreeLetters = $AllLetters | Where-Object {$UsedLetters -notcontains $_}
$FreeLetters | select-object -last 1