There is a twain scanner driver conflict with another scanner:

1. Disconnect the USB cable between the Macintosh and your Brother device.
2. Log on as "Administrator".
3. Delete any scanner by another manufacturer (Macintosh HD/Library/Image Capture/TWAIN Data Sources)
4. Reboot the Mac.
5. When the Mac has rebooted, empty the Trash.
6. Plug in the USB cable and try to scan again.

I am still not able to scan:

1. Disconnect the USB cable between the Macintosh and your Brother device.
2. Reboot the Mac.
3. Log on as "Administrator".
4. Delete the "Brother TWAIN.ds" folder (Macintosh HD/Library/Image Capture/TWAIN Data Sources/Brother TWAIN.ds).
5. Run the "Print Center" or "Printer Setup Utility" (from the Macintosh
   HD/Applications/Utilities/Print Center or Printer Setup Utility folder), Click on the model name of your Brother product in the printer list and remove it by clicking the "Delete" button.
6. Drag the "Brother" folder (Macintosh HD/Library/Printers/Brother) into the Trash bin and then empty the Trash.
7. Reboot the Mac
8. Insert your Brother CD-Rom and run the installation for the MFL-Pro software.
9. Once the installation reaches the Installation type click Customize and make sure Brother RSP and BrotherTWAIN have a check mark then click Upgrade and finish the installation.
10. Restart your computer at the prompt and plug in the USB cable.
11. Scanning should now work and you can reinstall any other scanners if needed.