Its all French to me :)
I found that number by higher or lower each decimal place and testing each time like a game puzzle.
Unfortunately although it formats fine without requiring a downgrade the second you try to do a file system restore it corrupts and requires formatting :(
I was warned...
I found the limit firmware 4.87 can handle, it's 1.65TB or 1649.267441663999989GB.
Not documented anywhere I could find so I thought i'd share here as its the first hit on google.
On a shucked 2TB Seagate Slim Portable ST2000LM007 I had to trim the drive with HDAT2/HPA to a max of 3221225472...