I use lightroom mobile on my iPhone and the photos sync through to the copy of lightroom on my desktop as they should. I'd like to be able to create a backup copy of these on the physical hard drive of the mac separate from the copy in lightroom mobile, in case anything happens to my iPhone - that way they'd be picked up on the mac's regular time machine backup (or I could copy off the raw images onto a DVD if I chose).
What is the best practice way to do this?
They are synced to your local harddrive. You can specify where in settings.
Just to be a bit more precise, on the Lightroom mobile tab in Preferences (Lightroom>Preferences on Mac, Edit>Preferences on PC), in the Location section you specify where the photos synced from LrM will be stored on your hard drive. By default they are stored in a .lrdata database in your user folder. I recommend changing this to instead store them with the rest of your photos. Changing this setting will change the location for future syncs. For past ones, use the Folders panel in the Library module to drag them where you'd like to store them. (Here's more information on how to use the Folders panel in Lightroom to reorganize your photos and folders.)