If you're on 2008R2 or 2012 you can use sys.dm_os_volume_stats to query disk information.

It would look something like this:

SELECT DISTINCT d.logical_volume_name

,d.volume_mount_point

,CAST(d.total_bytes / 1024 / 1024.0 AS INT) AS TotalMB

,CAST(d.available_bytes / 1024 / 1024.0 AS INT) AS FreeMB

FROM sys.master_files f

CROSS APPLY sys.dm_os_volume_stats(f.database_id, f.file_id) d

One small caveat: it extracts information only for the disks that contain a data or log file.

Hope this helps

Gianluca