Just thought I’d throw in my two cents (of course with inflation I guess it would now be three cents). I have been using a free app called Superduper, made by Shirt-Pocket, for 10-20 years.
The software is updated frequently, to keep up with newer versions of Mac OS, and it is compatible with 10.10 and later.
The main beauty of using Superduper is that it creates a bootable backup (on your external drive).
The latest version is 3.6.1.
The external drive needs to be formatted (using Apple’s Disk Utility program) as APFS (which is newer than Mac OS Journaled, extended, and is required with newer software. It only takes about 20 seconds to reformat to APFS.
My 2017 5K 27” iMac has 1 TB of solid state storage. I have been buying the western digital, My Passport for Mac, hard drive, in the 2 TB size, for around $70. It has a 3 year warranty.
The app can be downloaded and run for free forever, but you have to do a full backup each time, unless you send them around $29 to enable the Smart Backup feature which saves tons of time (only have to pay once).
In addition to monthly Superduper backups (with alternating hard drives) I also have another external drive running Time Machine constantly. But time machine is not bootable like Superduper.
Superduper has saved my bacon a number of times. When an older iMac internal hard drive bit the dust. I had a new one installed, then simply attached the Superduper hard drive to the USB port and told Superduper to copy the backup over to the new internal hard drive. Several hours later all was well (may be faster if you have less data and applications to restore.
Hope this helps someone.