http://itknowledgeexchange.techtarget.com/powershell/next-drive-letter/

function get-nextdriveletter {            

$disk = Get-WmiObject -Class Win32_LogicalDisk |
sort DeviceId -Descending |
select -First 1 -Property DeviceID            

$letter = ($disk.DeviceID).Substring(0,1).ToUpper()
if ($letter -eq "Z"){
 Write-Host "No more drive letters available"
}
else {
 $nextletter = [char](([byte][char]$letter) + 1)
 $nextletter
}            

}