What size of external disk drive for Time Machine

I would recommend at least double your internal storage drives capacity, that way you can guarantee an older version of files will be available even if your internal drive is near full. If you need external drives for other purchases, use physically different drives, me, I wouldn’t risk my backup.

I wouldn’t recommend partitioning it for use with other things, if you screw it up, there goes your backup.

I would recommend using different types of media if possible, but if using only a single backup, an HDD is fine, but an SSD is best, as if dropped it is (much) more likely to survive with the data intact. Also an SSD will complete backups faster. But since you have an iMac, you would be able to leave an HDD plugged in all day.

TimeMachine takes control of the drive to more efficiently index/protect the backup. When you also have manually controlled general-use partition you can make errors with the TimeMachine partition, so it is best to have separate physical drives for different used.

True ... a strict reading of the rule can be expensive. But if you purchase 4 TB (as I did) only because you cannot upgrade later, and you plan on using 1 or 2 TB in the near future, you can replace backup later as prices go down. But I am expecting 1 TB to be minimum amount "in use" and needing backup.

TBH, I am excluding everything from a Time Machine backup except my home folder. I have also tried to exclude items from ~/Library but it's very complicated in deciding what to exclude or not (for example I think ~/Library/Caches can be safely excluded). I have also tried doing it manually using rsync, but the built-in rsync is like 20 years old, then you have the option of installing a newer rsync using homebrew, but then doing backups using scripts is cumbersome. I have also tried Carbon Copy Cloner but I always fallback to using only Apple's built-in tools and decided some time ago that Apple's tools are the most reliable.

Note: I am simply using Time Machine to keep copies of my work/photos/files in case my disk is crashing, I am not interested in having past images. For me a 1 TB disk for a 1 TB internal storage does the job.

The other exchange is probably confusing you, so ...

If you have a 4 TB drive, get a backup that is 2x the expected usage ... 1 or 2 TB used of the 4. By the time the backup drive is too small TBs will be cheaper.

As to "selective backup" ... do not. This backup is supposed to rebuild in the case of traumatic failure, including installed applications. Let TimeMachine or CarbonCopyCloner (I use CCC as it makes a fully bootable copy of your internal system including partitions) do its work without micro-managing. CCC is available for about $40 at Bombich.com.

That is true, I agree, if you are looking at over 2tb, you should probably just get HDDs for the cost, and just be more cautious. You can get 8TB HDDs for the cost of a QLC 2TB Samsung QVO 2TB SSD, so if you need bulk, an HDD would have to be the way to go. I wouldn’t recommend an SSHD however, you won’t see any meaningful performance gains, and I would just see it as something to go wrong. Seagate’s current SSHD’s only have 8GB of flash, which means you’ll only see fast writes for potentially up to 8gb of data transfer, but if doing a backup, that’s irrelevant, as it will be too small to make a meaningful difference.

SSDs up to 2TB, HDDs for beyond…

Lida Verner wrote:

So TimeMachine won't create a bootable copy? Then again, a bootable copy is more for a catastrophic hardware failure, correct?

Correct on the first and mostly correct on the second. I say that because there can be some instances where everything is functioning fine but some update to an app for instance is unusable and a deadline is looming, you can quickly reboot to the clone and get back to work and deal with the problem later. Restoring to a previous state from Time Machine to a point before the change can be possibly be quite time consuming.

TimeMachine should be fine for backing up my files but they will be written over as things change?

Eventually, as the backup drive fills up, Time Machine will trim older backups to make room for new backups. One reason for your most important data you should have a separate backup on a different device. However, there will always be a complete backup of your current state of your machine. If for some reason you are trying to recover some file you deleted two years ago, if the Time Machine drive is not large enough, it could likely be lost.

As an aside, as a rule, you should have at least to copies of your most important data backed as a single backup could fail.

woodmeister50 answered most of your questions. Both are good for recovery from catastrophic events. And with these new slim all-on-the-logicboard systems, a new system with no attempt to transfer your data is a common response from Apple. You need a copy that can over-write the new system with your last working image (including installed applications).

As woodmeister50 stated, clones are good at "work for now, fix it later" scenarios. But you also can tell that running clone to "clone back to internal", or you can just copy back individual files. TM can recover, but needs a working system. Make your choice and learn usage before the disaster hits.

Both work, and many keep one (or more) of each. Some even rotate copies off-site (friend's house) in case of house damage or burglars.

You can get any external drive to do Time Machine just make sure it’s a SSD (state solid drive) for better performance and speed 500 gb to 1Tb should be fine

A mechanical drive is fine for a ™ backup. Speed is not that important at all.

You want the backup drive to be at least two times the size of the startup disk.

Apple recommends twice the size of the internal drive, but I have a 1 TB internal storage and a 1 TB external SSD drive and I'm doing fine.

If you keep the same backup and start using close to the 1 TB, then the "old copies of files" will eventually accumulate and there will be no room for "old copies of files" anymore.

The problem is... let's say you get the maxed out Macbook Pro, which is 4 TB, then you need a 8 GB external SSD drive. That would be very very expensive.