I'm probably going to throw a monkey wrench (spanner, for the Brits) into this, but....frankly, I have never quite trusted Time Machine and for my purposes don't feel that it is the best way to do backups (although certainly it IS convenient). When I run a backup of what is on my computer I actually only back up the things which are important to me: Documents, Pictures, Movies, etc. I plug in the external drive and open/create a folder with whatever the current date is and then I copy each of my folders into this: Desktop, Documents, Music, Movies, Pictures...... These are the files which are most important to me, and in the end if a computer dies and I need to replace it, it is very easy to simply use the backup external drive to then place (copy) all of the same files right into the new machine. That way the OS doesn't matter as much, the configuration of the machine doesn't matter as much, as in the end it's the files that are important to me.
One issue that I have with Time Machine is that my understanding of how it works is that it eventually ages off -- deletes -- older files and folders, which is fine if they are truly no longer important to the user -- but what if they still actually are? In some situations they well could be, especially if there are a lot of photos involved. Not too long ago I pulled out older external drives and transferred their contents to new drives with current ports; if I'd been relying on Time Machine all this time many of those (primarily photo) files would not even have been available.
Another reason I do this is that through the years along with changes in ports there have been other changes in hardware: i.e., "spinner"/"platter" hard drives are increasingly being replaced by Solid State Drives (SSD). So this is fine and dandy until one realizes that on the older machine one is about to replace there is a pretty full 1 TB "platter" drive but the new machine sitting on the desk has an SSD with significantly less capacity. Decisions have to be made about what should be simply stashed on an external drive and what needs to remain on the active computer. Apple is releasing all of its current lineup of notebook computers with SSDs and for people coming to these from a desktop (say, an iMac or a Mac Mini) this will be challenging. For those coming from an earlier MBP or other Apple notebook it could also be challenging if that earlier machine had a larger capacity storage than what is available at a reasonable cost now on SSD.
OK, I'm rambling on and on...... Anyway, bottom line here is to take into consideration just what it really is that you want to save or have already safely stored on another drive if something goes awry with the computer you're using.......
As for the time factor, yeah, doing this can be time-consuming, but I just get things set up and copying from the computer to the external drive and then go do something else for a while. Once the initial copying has been done, then on future updates it's not all that hard to simply add anything new that has come in since the last update, but, yes, that can also be time-consuming in a different way.
[doublepost=1488325030][/doublepost]Adding one more thing...... Every now and then I will check on my drive's capacity by simply highlighting my drive icon on the desktop and then "Get Info," and if all looks reasonable there, that's the end of it. If there seemed to be something seriously out of whack then I'd probably first run OnyX and then if the issue didn't seem to be resolved, run Daisy Disk to see just how things are spread out on the drive. Both are valuable tools to have on hand.