Ben,

I believe that the first cell of a column dictates it's format on an append / import.

This might be the ultimate cause. The experts in this forum will probably know for certain.

I have experienced this quirk and have taken to use automation to open a speadsheet and save it as CSV before append / import into my tables. Seems to have relieved some of the angst of day to day processing.

Cheers,
Vince