Time Machine keeps stopping at about 56%

FWIW. I've seen this when there was one or more Time Machine local snapshots on my Mac that is/are corrupted.

Once I removed them and started a manual TM backup, the process completed successfully. Might be worth a try in your case.

The easiest way to "remove" them is to temporarily switch TM from "automatic" to "manual" backups. You can do this via System Settings > General > Time Machine > Options ... > Back up frequency.

Leaving it in manual, for about 10-15 mins, should do the trick. I suggest that you leave it in manual and run a manual backup to verify that it completes successfully, before resetting it back to automatic.

Ref: About Time Machine local snapshots - Apple Support

Overall SMART status tells you whether you have an overwhelming number of bad blocks, which suggests your drive might be getting really old.

If one of your source drives has a few Bad Blocks, that is not carried into the overall status. You can read out the detailed SMART status with tool like DriveDx. The free version will probably work for this purpose.

https://binaryfruit.com/drivedx

Counters you will want to look at closely are things like current pending sector count, which tells you how many blocks are known BAD, and are waiting to have a spare block substituted.

Hi, did you ever find a solution to this problem? I am facing the same problem now and I have tried pretty much every suggested solution I could find on the internet. I am still unable to create a new backup.

do you have a Bad Block on either the source or destination drive?

on a rotating magnetic drive worst-case re-reading can take a quarter minute per bad block.

Disk Utility Repair disk function reads and repairs ONLY the directory, no data blocks are even read.

¿how's the SMART status of the drives involved?

If tracking down Bad Blocks does not help, there is a way to read Time Machine logs, and see what it was doing when it gave up. But to do it properly requires special tools.

I am not current on what tools are needed.

I hope someone else who knows will chime in.

was the Mac allowed to Sleep while the backup was in process?

many aftermarket drives do not transition in and out of sleep, instead just disconnecting.

For time machine, the assumption is that you drive will be reconnected later, when you get around to it, so it does not always throw an error when that happens.

Does Finder still show the drive as Mounted (not "ghosted") ?

No, my macbook is never allowed to go on sleep mode automatically, and the Finder shows the drive as mounted. I also ran first aid on the drive from disk utility and everything seemed to be okay with it. I retried the backup after first aid and it still had the same behaviour: stopped at around 55%.

Disk Utility First Aid should rightly be named Disk DIRECTORY First Aid. it does not read ANY data blocks at all.

You could have Bad Block at the 56 percent mark, and Disk First Aid can not tell you anything about it.

that was posted above. Did you read from the beginning?

Overall SMART status tells you whether you have an overwhelming number of bad blocks, which suggests your drive might be getting really old.

If one of your source drives has a few Bad Blocks, that is not carried into the overall status. You can read out the detailed SMART status with tool like DriveDx. The free version will probably work for this purpose.

https://binaryfruit.com/drivedx

Counters you will want to look at closely are things like current pending sector count, which tells you how many blocks are known BAD, and are waiting to have a spare block substituted.

I have the same issue. Everytime I try to back up it goes to 50-60% and then stays there for hours, before it either disconnects or returns to state the last successful backup (which is some time ago now). This is also the case when I try manual backup. My Seagate Expansion drive is about one year old and has lots of space.

did you do Tesserax suggestion of:

changing to manual backups (i.e., turn off auto bankups),

wait a quarter hour,

then initiate a Manual backup with 'Back up Now'?