Reasonable external drive size for Time Machine for a 512 GB MBP?

Given your present usage a 1 TB drive would certainly be adequate, but 2 TB drives cost only a few dollars more.

Would this be a good strategy?

The best strategy is to have a sufficient number of backup drives so that when one fails, you simply throw it away and replace it with a new one at your convenience.

Reasons: They all fail eventually, and Time Machine usually gives you enough advance warning when it alerts you that it could not back up to that drive. So, reformat it and place it back into service. A year or two or three later it will fail again, so you repeat: reformat and place it back into service Eventually that drive's failures will become intolerably frequent, so you discard it without a second thought.

Meanwhile, you have been using at least one or two other redundant hard disk drives that haven't failed yet, and you are never without a backup.

Corollary: don't buy SSDs for TM backup devices. Not only are they too expensive to maintain three or four or many of them, SSDs almost never give any advance warning of failure. They simply die, and when that happens they can't be reformatted. You throw it away just like a hard disk drive, except the HDD might limp along for another couple years.

Lastly, speed is utterly irrelevant for Time Machine. The speed constraint is Time Machine's activity, not in read/write speed of the backup device. "Slow" 5400 RPM HDDs are perfectly ok.

Twice the size of the SSD on your computer should be more than enough. I have a 1TB MBP and my Time Machine Drive (a Samsung T7 SSD) is 2TB. Whatever you use, backup that computer. People come here every day crying over lost data after something went wrong and they had no backup.

Because there is one obscure case after a software update where you will need to store more than twice your data (counting new, old and history) the "company line" is at least 2.5 times the size of everything you might need to back up.

The minimum is you should start right now with whatever drive you have, at least as large. as your data.

You can always add another drive at any time, and time machine will create a new stand alone backup on it starting with current data (it does NOT copy over the old). Then Time machine will alternate backups -- every other backup goes to every other drive. Since each drive is an independent stand-alone backup, the old drive can be removed at any time.

By design, Time machine works at low priority in the background. Do not pay extra for a Time machine drive that is FAST.

Nearly all my TM backup drives were pulled from discarded Windows PCs. In other words they were free. An enclosure or SATA connector brings the cost to about $10.

Speed is irrelevant.

It's your money though.

Is any drive completely removed from failure? Your statement could be don't buy a computer with SSD drives. I've been using a Samsung T5 SSD drive for more than 3 years and while not inexpensive, it's been extraordinarily reliable. I bought the T7 2TB drive as my Time Machine Backup for my new MacBook pro and while by no means inexpensive, to me, it's worth every penny and I have every expectation it will stand the test of time. But then, I also pay for Carbonite Backup for photos and documents, so redundancy is for me a good thing.

Consider the fact even a modest capacity TM backup drive maintains several multiples of the millions of files its source volume contains.

Result: they will fail commensurately faster.

If you can afford multiple redundant SSDs, great! They are superior in every respect, but that superiority is wasted on TM.

They will eventually become cheaper than hard disk drives, and I won't miss them a bit.

+ 1 !!

The "cloud" of any flavor, although the latest "thing", is sorely overrated. Personally, I would never trust any of my precious photos or sensitive personal information to a nameless online server which can and will be hacked into sooner or later.

Rather than using TM, I am "addicted" to and prefer bootable clones and am staying on an older OS version on my main "work" machine for two reasons: it still allows me to use some pricey older apps (which only get used occasionally now and therefore not worth spending $1,500+ to replace) and that makes using CCC to do regular backups still a one-click operation for a bootable clone.

I maintain a minimum of 2 clones for every OS version I have plus 4 separate drag + drop backups of my Documents folder which has anything/everything I need - all on 6 hard drives (some 7200 rpm and some SSDs), all are partitioned.

2TB

Or 1TB depending on your budget and expected size of maximum dataset through time.

It's a personal choice only you can make.

I am so glad that your SSD drives have not failed.

SSD drives, WHEN they DO fail, tend to fail suddenly and completely, and without ANY warning.

When Encrypted drives and Striped RAID arrays (of any type) fail they leave NO SURVIVORS.

So it is essential that you have backups.

In addition, with MacBook Pro drives soldered to the Mainboard, mainbord failure often takes out the boot drive as well.

In addition to daily-use drives failing, backup drives also occasionally fail. Some die from old age. in the long runYou really need more than one backup.

Any drive can fail. But I do believe in the old adage, you get what you pay for. Buying a higher quality drive for Time Machine backups, at least for me, makes sense. And then having a service like Carbonite ensures my most important items can't be lost.

I do not share your respect for online storage.

"On the cloud" is great for sharing photos, but is not a viable backup solution for everything you have. The stuff is not under your control, and is subject to sloppy handling, arbitrary changes in policy, theft, data loss [are they making frequent backups using best practices?], and bankruptcy of the company that holds it. It can easily take three days to restore it at ordinary Internet speeds.

That's fine. You don't need to share my respect for online storage. I respect Carbonite as a service and I find it worthwhile to pay for it.