Close this one up, after two excellent responses confirming your use of the internal SSD. Up until a few years ago, I operated all my computers on 120GB SSDs, mainly due to the expense. I just made sure to store all data elsewhere. While I don't always agree with Fishrrman, I know he is a very experienced commentator on this forum. My instructions would be identical to his. Boot from the internal, but make sure there's less than 80GB (preferably<60) to be sure your macOS has room for a swap file. I use SuperDuper, but have used CCC, both work well. The later is $40, a very fair price, but SuperDuper will work the basic erase and clone for free. Then, for all the features, it's only $28. When I help friends or clients, downloading SuperDuper is usually the first step. I also recommend you create a second clone on a separate data drive, as over the last decade, I have needed both clones. Then, rotate the backup between clones, but be certain to boot from the clone to TEST it. The Shirt Pocket (company name for SuperDuper coder) guys, Dave, mainly, has helped me numerous times. The diagnostics are built in and you can send them and get back an email with directions. They have never let me down. I also use the optional "copy newer" feature for data backups, but mostly I use the Smart Copy feature which makes updating clones much faster than the normal (free) erase and clone. As an example, I have a Big Mac and a MBP. My cMP only goes to Mojave 10.14, so I also keep a copy of High Sierra 10.13 and two clones of each as well. I have a set of clones on an SSD (internal or external) and another set on an old HDD external. On the MacBook Pro 2012, I have a similar routine using Mojave and Catalina with a 500GB boot SSD and swapped out the DVD for a 750GB 2.5" HDD to store a clone and data.
