ideas for storing/logging a LOT of measured data

Guys - been following this one for a while, just got back to a project I'd put aside for a while..

I've got everything working with SDFat, but I'm running into what I'm sure is a boneheaded mistake on my part. My data structure is 12 bytes wide, and I'm writing to a 512 byte buffer before flushing to the card. When I look at the data on the card, I see my 512 bytes written out perfectly, but for some reason I'm getting an additional 8 bytes written to the file as well?!

Obviously this is not what I want - I want a file of 12 byte records, and the "extra" 8 bytes causes an offset. Any thoughts?

sizeof(MyBuffer) is 512, and what I get in the file is
512bytes + 8 bytes + 512 bytes + 8 bytes, etc... The "extra" bytes are all 0x00.

EDITED : Ok, so I fail at basic math - problem solved!