Például a szabad kapacitást így teheted változókba:

Dim sPath
Dim drv
Dim oFs
sPath = "C:"
Set oFs = CreateObject("Scripting.FileSystemObject")
Set drv = oFs.GetDrive(oFs.GetDriveName(sPath))
bytes = drv.FreeSpace
kilobytes = bytes / 1024
megabytes = kilobytes / 1024
gigabytes = megabytes / 1024
kilobytes = Round(kilobytes, 2)
megabytes = Round(megabytes, 2)
gigabytes = Round(gigabytes, 2)

(By Google)
http://www.bigresource.com/VB-Get-drive-size-space-sQzGt6bP6f.html#pmrhM7P3cp