If you're certain of the mapping between the physical drive and the C: drive, you could fudge a relationship. Doing it this way is based on an assumption, so keep in mind what they say about "assume".
With the assumption caveat in mind, you could try something like this:
Select Top (1000000)
tblAssets.AssetID,
tblAssets.AssetName,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssets.IPAddress,
Cast(Cast(tblAssets.Memory As bigint) / 1024 As Numeric) As RamGB,
tblAssets.Processor,
tblAssets.Username,
tblAssetCustom.PurchaseDate,
tblAssetCustom.Warrantydate,
tblAssets.OScode,
tblFloppy.Name,
tblFloppy.Model,
Cast(Cast(tblFloppy.Size As bigint) / Power(10, 9) As numeric) As SizeGB,
Cast(Cast(tblFloppy.Size As bigint) / Power(2, 30) As numeric) As SizeGiB,
tblDiskdrives.Caption,
tblDiskdrives.FileSystem,
Cast(Cast(tblDiskdrives.Freespace As bigint) / 1024 / 1024 / 1024 As numeric) As FreeGB,
Cast(Cast(tblDiskdrives.Size As bigint) / 1024 / 1024 / 1024 As numeric) As TotalSizeGB
From
tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Inner Join tblFloppy On tblAssets.AssetID = tblFloppy.AssetID
Inner JOIN tblDiskdrives ON tblAssets.AssetID = tblDiskdrives.AssetID
AND tblDiskdrives.Caption = (CASE
WHEN tblFloppy.Name LIKE '%DRIVE0' THEN 'C:'
ELSE ''
END)
Where
tblAssets.OScode LIKE '6.3.9600S'
And NOT ( tblFloppy.Model LIKE 'OCZ%'
OR tblFloppy.Model LIKE 'Kingston S%'
OR tblFloppy.Model LIKE '%Solid State%'
OR tblFloppy.Model LIKE '%SSD%'
OR tblFloppy.Model LIKE '%NVMe%'
OR tblFloppy.Model LIKE '%LITEON%'
OR tblFloppy.Model LIKE '%MTFDDAV%'
OR tblFloppy.Model LIKE '%Sandis%'
OR tblFloppy.Model LIKE '%MZ-5S71000%'
OR tblFloppy.Model LIKE '%pny%'
)
If the simple assumption of physical drive 0 = C: doesn't hold true, you could expand the conditions to match your setup, e.g.
CASE
WHEN AssetName = 'xxx' AND tblFloppy.Name LIKE '%DRIVE0' THEN 'C:'
WHEN AssetName = 'yyy' AND tblFloppy.Name LIKE '%DRIVE1' THEN 'C:'
etc.
END
It's a kludge, but it may do the trick for you.