Thank you for the post and I apologize for the delay.
Has the file been modified since the last backup was created?
The following is from KB10240, Frequently Asked Questions about Progressive Backups:
What are Progressive Backups?
A copy of a database file created by FileMaker Server applying only the changes that have occurred during the save interval.
How are progressive backups different than normal backups?
With progressive backups, FileMaker Server starts by creating a full backup of all hosted databases.
After the initial full backup is complete, the Database Server only copies the changes from the hosted file to the progressive backup folder. Progressive backup can run more quickly than a scheduled backup, with less impact on server performance.
How many copies of my files does FileMaker Server keep?
Progressive backup essentially keeps two copies of the backup files: a time stamped copy that is available for you to use as a backup, and an in-progress copy that gets updated with the accumulated changes.
Some additional knowledge base articles that may be helpful:
Thanks for your response, TSF. I thought I had the problem solved last week, but it has reoccured. While troubleshooting by looking into Filemaker logs and system logs, it seems both Filemaker Server and OS X are complaining about the thumbdrive I'm using for the backup device. Maybe it's too slow for the backup process or may just be a bad drive. Anyway, I'll try another device and see if that solves the problem. Again, thanks for your help.