csv import resets default category

[CrystalPhoenix18]

I use a csv import to update quantities and prices to coordinate between my bricks-and-mortar POS program to my Prestashop installation.  I use three fields:  the ID (from my Point of Sale program at my bricks-and-mortar shop), the quantity, and the price.  When I use this, the default category for all my products resets to "Home".  I have spent a lot of time and trouble setting the default category for each product (I have over 10,000 online) and now I have to do it all over again - and each time I update the quantities it all gets changed back to "Home" again.

Any suggestions for how I can keep the default category intact?  The csv file I generate from the POS has all the 18,000 products in my shop on the list, and to set the default category for each item is impossible, especially since a product can be in multiple categories.

Thanks for any help!