Hmm. I read in my copy of ATA7:
6.16.55 Words (103:100): Maximum user LBA for 48-bit Address feature set
Words (103:100) contain a value that is one greater than the maximum LBA
in user accessable space when the 48-bit Addressing feature set is supported.
The maximum value that shall be placed in this field is 0000FFFFFFFFFFFFh.
Support of these words is mandatory if the 48-bit Address feature set is supported.
Do you read differently?