Please use the following report:
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Lastseen As [last scan],
Sum(Cast(Cast(tblDiskdrives.Size As BigInt) / 1024 / 1024 / 1024 As Numeric))
As [Total size (GB)],
Sum(Cast(Cast(tblDiskdrives.Freespace As BigInt) / 1024 / 1024 /
1024 As Numeric)) As [Free in GB]
From tblAssets
Inner Join tblDiskdrives On tblAssets.AssetID = tblDiskdrives.AssetID
Group By tblAssets.AssetID,
tblAssets.AssetName,
tblAssets.Lastseen
Order By tblAssets.AssetID