Tracing real-time filesystem calls by process?
This was, ultimately, the best way I could find to do it:

sudo fs_usage -f filesys | grep backupd


If I wanted to stop the flow:

sudo fs_usage -f filesys | grep backupd | more


And if I wanted to output to a text file:

sudo fs_usage -f filesys | grep backupd &gt;output.txt


I largely found this information here at AskDifferent. I then used the manpage for the command to piece together my commands above.

Hope that helps someone.