I ‘ve been asked about Lightroom best solutions for backing up and I though i’d share my method! This graphic best explains the setup. All my RAW images are uploaded to an external Drive ( Drive 2 ) that way my computer hard rive is clear of all large files. The Lightroom Catalogue which automatically stores on the computer hard drive. I have set create a catalogue backup every time I close down Lightroom. This can be set in Lightroom’s preferences and I have this backup going to the external drive alongside the RAW files. On another Hard Drive ( Drive 3 ) I use a piece of software “SuperDuper” to copy everything on Drive 2. This gives me a mirror of Drive 2. I also have a backup of the computers Hard Drive which goes to an external Hard Drive ( Drive 1 ). Let me know if you have other solutions that work for full belt and braces approach to backing up!! Mark
Blarney Photography Club 
My method is similar but I use Crashplan, Backblaze and also a backup script.
Like you, my photos are on an external drive while the Lightroom catalogue is on the internal drive.
Once a day my backup script runs, copying the catalogue to the external drive where my photos live. It also copies the catalogue and photos to another computer.
I use the free Crashplan app to backup the catalogue and photos to another external drive once a day. It creates versioned copies of files, just on the off chance that the catalogue or other files I’m working on become corrupted somehow.
I also backup everything “to the cloud” using Backblaze.
I’m still concerned about “bit rot” happening (that’s where a single bit in a file randomly flips from 0 to 1 or 1 to 0, because a cosmic ray hit the harddrive at the wrong moment, unlikely but I have lots of bits of data so the chance increases..) , which is partly why I started using Crashplan, but with the number of files I have it’s impossible to keep track of everything manually.
I also use Apple’s Time Machine. but not for my photos.