Disk failure?

<< I will attempt to make a bootable clone on another disk. >>

Cloning the ENTIRE drive may be a frustrating waste of time and energy.

Fully 50 GB or more and more than 350,000 files on that drive is invariant MacOS files, which can be re-created EXACTLY as they were by re-installing MacOS.

My recommendation is to copy EXACTLY the /Users folder, and the files in it, which are your treasured files.

if it won't boot, that adds weight to the argument that the trouble is in macOS files.

Run First Aid multiple times. If the same error remains, then you will need to erase the whole physical SSD, followed by reinstalling macOS & restoring from a backup. First Aid just cannot fix some file system issues (it has always been this way for decades).

You may first want to check the health of the SSD by running DriveDx (free trial available). Post the complete text report here using the "Additional Text" icon which looks like a piece of paper. Sometimes this health report may reveal if the cable is part of the problem because the internal hard drive SATA Cable can fail in multiple ways (personal experience supporting my organization's Macs).

Plus some SSDs are just not compatible with certain computers and even some name brand SSDs can be complete pieces of junk in regards to performance even from the same company (Crucial MX500 is good, the Crucial BX500 is absolute junk).

jacdig wrote:

I have a MacBookPro 9.2 13" mid-2012, with an upgraded SSD.

At some point lately it started freezing and behaving strangely.

I took out the disk and put it in an external case. I connected it to another machine to check it and repair it.

Disk Utility sees it a "USB External Physical Disk", containing an APFS Container (named “Container disk3”), this one in turn containing the APFS Volume named ("SSD 1TB").

Running First Aid on the external physical disk reports no errors.

Finder shows files and folders on this disk.

If selected as startup disk, it will not boot.

I understand that the disk is broken and needs to be replaced. I wonder how safely I can access it to copy the files.

You do no say what "another machine " or the associated macOS...(?)

typically you would want the same era/ specific Disk Utility tied to the same macOS for repairs for best results.

If it does not boot externally— then I would say yes you have an issue.

Replace the drive and SATA cable at the same time— if you intend to keep using the 2012...

or erase/reformat/initialize as new and try again installing a macOS.

SATA cable is a known source of failure, specifically the 13" mid-2012. Could it lead to corruption of the drive, I suppose so—typically you have some indication there are intermittent generic faults occurring...

SATA replacement 13"MBP mid 2012

https://eshop.macsales.com/item/Apple/8211480/%20%2013%22%20MacBook%20Pro%20SATA

Not advised to wait for a catastrophic failure to get the backup religion—

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.

More than one device, more than one backup methodology.

If no backup— then try and salvage what you can if you have access—what else are you going to do, safe or not safe seems rhetorical...