Time Machine not backing up both internal hard drives on 2011 iMac?

Question

I have a 2011 iMac with two internal hard drives---a 2TB HDD and 256GB solid state. 
I plugged in a 3TB FW800 external to use with Time Machine. 
Based on adding up the used space from both drives as seen in disk utility (totalling 1537.36 GB), it doesn't appear that Time Machine is backing everything up (total backup of 1.49 TB).
Any ideas?
Thank you,
Taylor

Answer 1

Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the page that opens.
Drag or copy — do not type — the following line into the Terminal window, then press return:
sudo tmutil compare
You'll be prompted for your login password, which won't be displayed when you type it. You may get a one-time warning not to screw up.
The command will take at least a few minutes to run. Eventually some lines of output will appear below what you entered.
Each line that begins with a plus sign (“+”) represents a file that has been added to the source volume since the last snapshot was taken. These files have not been backed up yet.
Each line that begins with an exclamation point (“!”) represents a file that has changed on the source volume. These files have been backed up, but not in their present state.
Each line that begins with a minus sign (“-“) represents a file that has been removed from the source volume.
Files that you’ve excluded from backup, or that are excluded automatically, are ignored.
At the end of the output, you’ll get some lines like the following:
Added:
Removed:
Changed:
These lines show the total amount of data added, removed, or changed on the source(s) since the last snapshot.