time machine - how add an external ssd in my mac backup?

Would suggest purchasing another External Drive and Dedicate it to only Time Machine Backups.

Make sure the external drive has at least 2 1/2 to 3 x Greater Drive Capacity than the Internal Capacity of the Computer

TM Backup will what to control the Entire Drive and mark as Read Only.

If the user puts all the " Eggs in 1 basket " and Drives do fail, Scrambled Eggs is what remains.

To really protect you important and nor replaceable Personal Data >>

Use the Rescue Plan consisting or 3 Backups using 2 different Methods and 1 Off Site incase of Natural or Un Natural Disasters

The above is predicted on each Backups / Each Method is done to a Separate & Dedicated Drive.

I have TM backup my internal boot drive and two external SSD:

You should have no problem backing up your boot drive and the EHD with TM.

The TM drive should be about 2 ½ to 3 times the size of the drives being backed up. I find these OWC drives to be excellent for that job: OWC Mercury Elite Pro. They are very reliable, reasonable prices and have excellent customer support and warranties behind them.

Following the suggestion from @ Barney-15E, technically should work fine.

I have never used TM Backup to backup Both Internal Drive and include an External Drive

Maybe I making this more complicated than it is ?

As long as the External Drive ( Photos ) is attached when the TM Backup is performing the Scheduled backup it should work.

But, this I do not have the answer, if the External Drive ( Photos ) drive is Not attached when TM is running, what happens when it looks for this specific drive and it is Not there ?

That is unknown to me and perhaps the more experienced Contributor has insights ?

Then, since the Photos External is included in the Regime of TM - the Snap Shots TM Macks Before they are transferred to the Destination Drive would by nature be considerable Larger than a Normal Snap Shotfor Only the Internal Drive.

The above is unfamiliar to me, also.

EDITED

Jumping off on P. Phillips questions, I hadn’t considered the APFS changes, so I should caveat that I don’t know if it still works. I haven’t seen any indication that it doesn’t. I also should point out some observations I have of doing a TM backup of an external.

On the old HFS backup scheme, I had TM backing up my external with Photos Library. I had a need to restore the Photos Library drive (drive issues). I found it difficult to restore that drive, so I now use Carbon Copy Cloner to back up it and a “Media” drive.

I have since spent some time investigating restoring backups of external drives and don’t think it is as difficult as i previously thought. You need to name the new, blank external as you did the one you backed up, then restore the external from the Time Machine interface. The confusion arises because the External may not show up in in the timeline if you have a backup selected where there were no changes to the drive. There is no way to tell it to, “restore the external.” You can’t even do that with the startup drive anymore. I also don’t think you can Migrate it from the backup when you migrate a user. How to restore the external doesn’t seem obvious, and panic doesn’t help in understanding.

With CCC, I just clone the backup to a new drive.

... do you agree?

That's what I concluded.

I was able to restore my external drive from Time Machine, but it just wasn't as clearcut as I would have liked. It was my pre-conceived notion of how I would have expected it to work interfering with my just thinking about it as just another file or folder to restore. The problem was where to restore as it seemed to want to restore the Photos Library to the startup drive in my home where there wasn't room.

CCC does have a cost, but I thought it was worth it for the ease of use.

It may be worth the effort for some to experiment with backing up and restoring the external. It may not be as difficult as I make it sound. I did help someone restore to an external, but it took quite a few back-and-forths to figure it out. That's where I discovered how it doesn't necessarily show the external on every backup set.

Unless mistaken - there ie a 30 DayTrial Period and Paid for to continue to use

Several Links to CCC and some specific issue to read up on

https://bombich.com/features

https://bombich.com/kb/ccc6/macos-ventura-known-issues

https://bombich.com/kb/ccc6

https://bombich.com/kb/ccc6/cloning-macos-system-volumes-apple-software-restore

LuigiCelle wrote:

Thanks guys. The aim of my original idea was to simplify the backup management but, as far as I can understand, maybe the easiest and safest solution is in fact using TM to backup the mac and using CCC to backup the external libraries... do you agree?

I also agree with the others, Time Machine is best for keeping versioned backups going back in time of the internal boot drive. It CAN also back up additional external drives but the logistics of restoring are more complex (as Barney noted). In my daughter's business, which makes use of many external drives for certain data, those external drives are backed up using SuperDuper, which like CCC, is a cloning program. There is a free version of SuperDuper, but the paid up version has additional convenient features. As with CCC, the paid up version is worth it, they are not expensive. Restoring from these clones is easy: you can run the cloning program in reverse to restore the entire drive, or you can use the Finder to just copy back what you need.

Make sure it is formatted in a Mac format and make sure it isn’t excluded in Time Machine settings.

Ditto >> Barney-15E