Using Time Machine to backup two internal disks onto one external HD

First of all, Time Machine will do all the work if you let it. Just leave the drive plugged in with the MacBook on and connected to power.

The Time Machine can only encrypt while both of those things are true. So if you have your MacBook set to sleep or log out after a certain amount of time it will stop backing up/encrypting.

Also definitely don't copy/paste your internal disks to the external. This serves no purpose and will only interfere with Time Machine. TM will automatically backup the entire internal drive.

Go to System Preferences > Time Machine and make sure you have both internal disks set to backup if that's what you want to do.

This is not the way to do it with TimeMachine.

Partition/Format the external disk with two partitions, Guid Partition Table, Mac OS Extended (journaled).

Then download a clone app (Carbon Copy Cloner or such), and install it on the startup disk. Make a clone of the Startup disk in one partition (answer yes to making the Recovery Partition) and of the other disk to the second partition of the external. The external clone of the Startup disk will be bootable.

Note: do not install any Seagate software, or if you already have, uninstall it: this software is very counterproductive.

Time Machine automatically makes the disk bootable and if it's not it will reformat it before using it.

CCC is great if you like finicking around and paying for it to avoid the nagware features and advertising.

Time Machine backups are complete backups of the system and are bootable and recoverable from especially if your main mac drive has Recovery HD installed as all new installs of MacOS have.

Also TM can manage backups from two different disks on one volume, you don't need separate partitions.

You could also use SuperDuper, which is cheaper, and a good tool.

CCC is a terrific app, though, and was free for quite a while. I can’t recall there being paid upgrades on a yearly basis. I rely on it and consider it well worth the cost. It will easily do exactly what you need, and Time Machine will not.

The best approach is to have different kinds of backups as each has its pros and cons.

Actually, I prefer using CCC over TM - I do not care for a machine (or software) to decide if/when I wish to back up. And, the bootable clones can do something TM cannot: they contain your entire OS system whereas TM has to "fetch" it; as far as I know, TM is not bootable in and of itself as you do boot into recovery. Which, when you need to reinstall an earlier OS version, will not work. Having bootable clones has saved me from disaster several times over the years.

And, Bombich is not "unidentified"; he is a well known and trusted developer (disclaimer: I am absolutely not involved with his company; I am simply a customer). I have no problem updating his app. You may not like it, but his app is well regarded here.

If you want to have TM do more than one backup on one hard drive - which is a bad idea, by the way - then you need to partition that hard drive first, so TM has its' own "space" which to fill up with the two different backups.

Why is it a bad idea? Because every hard drive will fail - it is just a matter of time. So, having all your backups on one drive would mean that you will lose all of them. Personally, I prefer having smaller (1 TB) drives, but more of them (I have a total of 4), each with several partitions, and have each OS version cloned twice on two different drives plus very important files/photos/etc on 3 different partitions.

As for CCC, I've only had to pay for an upgrade once in about 5+ years (when he went from v.4 to 5). Every other update was free.

am having difficulty getting it to backup the 270gb disc in addition to the 500gb drive

Another way to do this is by manipulating the exclusion list in

System Preferences > Time Machine ...

... so that your non-boot drive is not excluded. Then Time Machine will back up both in a routine manner, and nothing else 'special' needs to be done.

You would only need to partition the drive to use it on an ongoing basis for backing up two different Macs (not two different drives on one Mac). Otherwise, it does not present any special difficulty that would require partitioning.

"Sorry my reply is so delayed, my drive just finished encrypting the 500gb backup of my Macintosh HD. However, Time Machine does not appear to have settings by which I can specify that I want the 240gb startup drive to also be backed up."

- In case you want to upgrade to HighSierra, I propose to unencrypt the TM disk first: HighSierra has a different kind of encryption.

- The answer of Grant B-A is absolutely great: do it like that !!

What error messages are you getting?

carbon copy cloner will make a bootable clone of the startup disk automatically.

I'm not disputing that CCC makes a bootable clone. I used to use it. But why install 3rd party software from an unidentified developer when TM does it for you built-in.

It is different (and I use both). Bombich is a very respected developer.