Assuming your Custom5 list formats the drive captions to match tblDiskdrives.Caption (e.g. 'D:,E:,F:'), you should be able to add this to your WHERE clause:
AND CharIndex(tblDiskdrives.Caption, tblAssetCustom.Custom5) = 0
If the formatting isn't the same, e.g. if you don't include the colons ('D,E,F' or even 'DEF'), you could achieve the same effect with
AND CharIndex(Left(tblDiskdrives.Caption, 1), tblAssetCustom.Custom5) = 0