Today I started a fresh backup of my stuff (1.7TB of data). No small task. All was going smoothly as I headed out for a while, with speed of about 200MB/sec.
When I returned, I noticed and heard an excessive amount of disk activity, and speed had dropped to about 110MB/sec. This was troubling, because I reasonably expecting the backup to be nearly done.
The problem? Apple’s Spotlight, which shows up in Activity Monitor as “mds”. I don’t need my backups indexed, and certainly not while I’m backing up.
The solution: exclude your backup volumes from being indexed.