Network Brother scanner: change default scan destination

I hooked myself up with a Brother MFC-J840DW. I got the wireless print and scan functions to work: when I start a scan from my computer (using simple scan) I can save the scan only on the same HDD as my Ubuntu 14.04 is installe on. I want to be able to save to an additional HDD. How do I do this? Also, when I start a scan from the scanner directly, the scan is by default saved in /home/MYNAME/brscan. Again I would like to change the destination to be on the other HDD. Any ideas? Cheers