Double-sided printing with Brother 2170W?

You are missing a D in the printer's name. That stands for Duplex on Brother printers. If you do not have it, you cannot use the automatic duplex (double-sided) option, because the printer has not got it built in.

You have a good inexpensive printer, but it does not do automatic duplexing. You can print every other page by choosing odd or even in Paper Handling, turn the result over, and print the backs of the page, but your printer will not do this automatically.

It can't be done on a mac, but it's still really easy to print double sided. You just print all odd pages first, put that package of printed pages into the paper tray correctly and print all even pages. Works flawlessly for me every time.

If it's the same for your printer, you take the package of odd pages printed and place it in the paper tray face up with the top of the text on the right hand side of the tray. Test it out to figure out exactly how to place the pages in the tray. This will print the even pages on correctly. Note that if it's an older printer, you need to let the pages set before re-inserting them in the tray because they'll be curled, so the printer won't grab them properly. I think you should fan them out too because there's a lot of static holding them together.

Hope this helps!