I think you’ve covered everything I had to go through with mine. If you haven’t checked already here’s the thread of tested drives ‘Tested’ Hard Drives
Does the drive light up when plugged into the Tablo? Solid or flashing light? I think that when mine was flashing initially was when it didn’t recognize it. I did read in another thread that using a powered USB hub was OK… you might try using a powered hub if you have one.
Good luck! Did you open a ticket through their web site? I did get most of my info through that. I think they replied within a day or two.