rsync mirror of download directory

I have a download directory for NDM Secure+ to receive files from other system. I need to mirror this directory to another box, but need to skip files that are still coming in.
I have no control over file names or destination directories as they are configured on the sending side.

How can I keep rsync from copying files that are still in the download process.

I run a cron every 5 minutes to keep files synced, is there something I can add as an exclude for these files?

Thanks

This actually can be a hard problem. How are the incoming files stored? FTP ? SSH? shell script?

Can you run the command "lsof" ? If not, can you install it ? If you can run it, try:

 DIR=<dir-to-mirror>
 /usr/sbin/lsof -F nc  +D $DIR  |sed -n 's/^n//p;' |sort -u

You can output that to a file and had it to rsync via --exclude-from=FILE option.
Don't forget to put the output outside of $DIR and remove it after each rsync.

NDM is just SCP on steroids I think, so the files would be coming in via ssh.

The files actually have a header and trailer stamp and I was going to write a script to look for both before syncing. I was hoping I had just overlooked a basic configuration option.