I noticed that on some Seagate Barracuda drives, "power on hours" is reported X + 65536 hours but where a short self test will report X hours suggesting that the most significant bit is included as 1 for power on hours and 0 for self test power on hours, maybe 0x10000 is added to the power on hours, I have seen this happen on a few drives including ST3750640NS and ST3250310AS these are 750GB and 250GB drives