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
just going to add one that works for remote drive letters $computer will be the input and $driveletter would contain the next available drive on the remote computer
67..90 | foreach {if(((GWmi win32_logicaldisk -computer $computer -Property DeviceID).deviceID).Substring(0,1) -notcontains [char]$_){$driveLetter = [char]$_; break}}
might be able to shorten that but at that length its clear to see whats going on