Time Machine - what all is needed to make it functional?

Any external hard drive is suitable. Here is the list of possible disk types you can use: Types of disks you can use with Time Machine on Mac - Apple Support

That specific drive is just what Apple sells. You can probably find it cheaper on Amazon or just about anywhere else, or find another drive that is cheaper.

Nothing wrong with that one but any SSD is overkill for Time Machine. What's required is capacity and redundancy, not speed.

You'd be much better off buying three 1 TB USB hard disk drives for the price of that one. Twice the capacity and triple redundancy. Keep one of them geographically separate from the other two at all times and you will have a very robust backup strategy. The likelihood of a catastrophic, system-wide loss of all your data will have been reduced to a practical impossibility.

allan299 wrote:

I have read on this message board, if i understood it correctly, that one should have an external drive that is 2-3 times the size of the storage capacity of the computer.

It's not quite that straightforward. The source volume's capacity is not the defining factor.

A TM backup device requires enough capacity to store one complete backup of the source volume's contents, plus enough to back up any additional files added or modified since that backup, plus an amount of "overhead" that is a not easily calculated with any degree of precision. For example if your source volume contains 450 GB of data, the 500 GB Time Machine backup device you were contemplating might be minimally adequate—for one backup. If a significant percentage of that 450 GB were to change, for example following a major macOS upgrade, it would be inadequate since TM will not delete that one backup until the next one is complete.

The oft-cited "rule of thumb" you quoted is imprecise. What is it anyway, two, or three times the source volume's contents? Why two? Why three? How about two and a half? Is any less than two ok, or should there be more than three? Four or five times the source volume? To answer my own question, if you purchase a 4 TB backup device you'll be fine. Should you purchase a 6 TB backup drive? Sure, why not.

The simple reason for that oft-cited "rule of thumb" is that it is an answer that will not result in complaints when the user finds out whatever he bought wasn't big enough.

Once in a while you can find 6 TB external hard disk drives for less than $100. Buy one. Or three. You won't come back here telling me I was wrong.

allan299 wrote:

Is the exclusive Apple External Drive that is shown as accessories: the LaCie Mobile SSD Ext Drive ($170US):

That would not be a good choice. It is too small.

If you want to buy directly from Apple, any of these drives would be better: https://www.apple.com/ca/search/External+Hard+Drive?src=globalnav

Is one of the reasons for backing-up besides 'losing the information' with an upgrade/update gone bad, in order to be able to 'move all that is on one machine onto a newer model in the future'?

You can do that without a backup, though it might be easier to connect to the backup drive than the old computer.

Sure, it's both.

If it helps, imagine yourself in a situation one day in which you go to turn on your Mac and it just won't. No amount of key-pressing or incantations will bring it to life. Or, your office and all its contents including your Macs and their TM backup disks were destroyed in a fire. Your only recourse is to buy a brand new Mac, perhaps one released many years into the future that won't even boot Catalina. Does that fill you with dread? Of course it would, as it would anyone, but if you could recover from that hypothetical event with all your data intact and with little more than having to pay an insurance deductible, then you're doing ok.

You can boot up the old one in Target Disk Mode and connect the two with the appropriate cable/adaptor.

You can connect to it over your home network.

You can even directly connect the two with an Ethernet cable, assuming there is a port on both, but that is rare on new Macs.

Migration Assistant will not replace the newer Apple applications. You can choose not to move applications at all. That is what I normally do. I install my third-party apps from source.

The configurations (network, printers, etc) do get migrated, and I think the preferences do also. The system settings are also optional.

How to move your content to a new Mac - Apple Support

It shouldn't require any more space than what you are taking up already.

If you are running out of space, and the new Mac doesn't have significantly more space, you should go through your files in your home folder and copy ones you can archive to an external drive or delete them if you don't need them. Once you do that, back up with Time Machine and use that to migrate your user data to the new Mac.

For the files you archive to an external drive, make sure you back that up if you care to keep those files.

Optimizing storage will move large, unused files to iCloud, but if you are not using iCloud or you don't have enough storage space on iCloud Drive, then nothing will actually change.