FM Update Import

I haven't had a whole lot of experience with importing data into my files, but I thought I understood well enough how it worked.

Our fiscal data system (Microsoft Navision) has clients in the database with their own, unique identifier. My filemaker solution obviously has it's own unique identifier for clients. I created an extra field to store the the fiscal identifier because we will be moving data back and forth between the two systems.

I got an export from our fiscal system data that consists of their unique identifier and a SSN. Every client on the export had an SSN.

I start an import into my client file. I use the SSN field as the match field and set their unique identifier to go into the field that I created for it. All other fields are set to "Don't import", and Add remaining records is not set.

When the import is run, it certainly brings in all the numbers as expected, but what it messed up was it cleared MY unique identifier from every record that had a match. Needless to say that was NOT good, I had to restore the file from a backup, but after one day had passed, so a lot of data didn't make sense. I'm still cleaning up from that mess, and I don't understand why it happened on the import.

Would it clear the fields that are all indexes for that file for some reason? Like I said, those fields were not selected to be imported, I have no idea why it would have cleared them.