External HD used for backup won't mount

What you describe sound symptomatic of a failed external drive.

In Disk Utility select Show All Devices in the upper left drop down menu so that all the different levels of the external drive are shown. Try running Disk Utility First Aid on all the levels that you can. There is a small possibility that you will repair something at a higher level that will restore the "lost" partition.

You should immediately try to copy off the files that you can from that drive, from all partitions. That is to prepare for the possibility (likelihood) that it has failed.

You can also download DriveDX to check the physical condition of the drive. Disk Utility checks the integrity of the file system, which is the information on the drive that allows you to find files and folder and data. However Disk Utility does not tell you much about the physical condition of the device, except when it cannot mount it (or a partition) or it freezes when trying to access areas of the drive. DriveDx checks and reports on the individual SMART health parameters (e.g. read/write failures, temperatures, etc.) and can run short and long tests of the drive's physical health.

DriveDX would not enable file recovery from the bad partition, but it could confirm a physical problem with the drive. Physical mechanisms that fail are basically a lost cause for most users, while a "soft" failure (like a corrupted file catalog) can SOMETIMES be recovered from.

When there is a physical failure on one partition, the rest of the drive (SSD or mechanical HDD) is suspect as well.

For file recovery, you would need to likely take the drive to a recovery service which is expensive. If the failed partition is used only for Time Machine, recovery efforts are probably not worth it (because such recovery efforts are not 100% successful and if there are any missing pieces, the Time Machine backup will not function in Time Machine anymore) and you should simply obtain a new drive and dedicate it to Time Machine without sharing storage for other things on it. (This is what Apple advises under Big Sur.)

You absolutely should copy whatever you can from the other partitions that seem to still be accessible, they may also fail at any time.

catmannn wrote:

Can you share with us trusted third party recovery/repair software for situations Disk Utility cannot handle?

There are many opinions on this. Here are mine:

For a drive that won't mount at all, no matter what you try, you will likely need a professional recovery company, like DriveSavers. The cost can be hundreds to thousands of dollars. Sometimes the drives must be physically taken apart and the zeros and ones read directly off the platters or chips, which is excruciatingly time consuming and must be done in clean rooms.

For HFS+ drives that will mount but are corrupted, I have used DIskWarrior and TechTool Pro successfully when Disk Utility did not succeed. Sometimes Disk Utility will succeed when the others don't work. They use different techniques.

TechTool Pro and Data Rescue claim to work on APFS and HFS+. I have them but have only used them on HFS+ drives. I have heard that when APFS drives fail the chance of any recovery is very low, but if there is no backup and the data are important, people will try most anything.

Disk Drill also has a good reputation and claims to work on HFS+ plus APFS drives, but again, a failed APFS drive is much tougher to recover, the odds are lower.

DriveDX won't fix or repair a drive but it you can mount it, it can tell you what condition it is in and what is wrong with it, if it is a physical problem.

Can you share with us trusted third party recovery/repair software for situations Disk Utility cannot handle?