NO I think you're on the right track. The BIOS only sees 6 ATA ports.
I can boot XPEnology with 1, 2, 3, 4 etc drives and it will boot successfully and see each one except for the seventh. Boot with ThecusOS and I can see and use all 7 so something funky is being done in ThecusOS to see the 7th drive?
What is different is that ThecusOS sees the drives as drive 1, 2, 3, 4, 5, 6, 7
XPEnology labels them drives 7, 8, 9, 10, 11, 12
XPEnology refuses to see the drive in the bottom slot no matter what order I use.
Brad