Determine free space on "nonfixed" drive
Alternative could be to use PowerShell to grab the information. All you need is a few commands tied together:


$measure = Get-WmiObject Win32_LogicalDisk | Where {$_.DriveType -eq 5}
if ($measure.FreeSpace -lt 20 { "More logic"} else {"something else"}


You could add in the logic to measure for your limit (value is in bytes). Then use Send-MailMessage cmdlet to actually send out your message. Wrap all that up and then have a SQL Agent job call the script on a scheduled basis.