MacBook Pro Time Machine backup to NAS fails to complete

Are you using an antique NAS that cannot be upgraded to a version that supports SMB3?

if that pull-quote applies to your Synology NAS, then you need to discuss your problems with Synology support.

AFP deprecated:

The terminology around this issue is very confusing. You need to read carefully to understand exactly what is being retired.

When you back up to a Server, you use a Server communication protocol to talk to the Server. Examples of Server communication Protocols are Apple File Protocol (AFP), which should rightly be called Apple File [Server] Protocol. and Server Message Block (SMB) developed by Microsoft and revised and updated many times since it first use.

What is being retired is every version of Apple File Protocol (AFP), This is not to be confused with Hierarchical File System Extended (HFS+) MacOS Extended, or the relatively new Apple File System (APFS). AFP (being retired) is a very old Server communication protocol, NOT A Disk Format.

The main group impacted are those still using a non-upgradeable Apple Time Capsule.

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On the Backup drive, you use whatever disk format the Server uses for its native disk format. Connecting to a Server, you send ad receive just files’. Using the Server launders out the actual format of its drive, which is a "don't care"

As long as your Server can create and maintain a sparse bundle disk image file, that is all that is required. Disk Formats of drives for MacOS and Linux and many others are just fine.

A sparse bundle disk image is like an entire filesystem inside one single extensible file on the Server Host.

a possible problem with some NAS devices is that:

although they can make ONE sparse bundle disk image for one backup, they fail to properly make a second one without manual intervention.

What does the file structure look like on the NAS now? Did you get a second sparse bundle disk image created for the second backup?

When backups are made over a network (Ethernet or wireless), the Backups.backupdb folder is placed inside a rather odd container called a sparse bundle disk image.  A disk image is kind of like a disk-within-a-disk -- it has its own format, directories, etc. Thus the format may be different from the format of the disk it's on. 

-from this section of the classic illustrated guide to Time Machine by the late James Pond

How Time Machine Works its Magic

How Network Backups are Stored in Sparse Bundles

https://tommyang.github.io/pondini.org/TM/Works3.html

<< although it did not create a sparsebundle file but merely a copy of my files and folders. >.

that is expected operation for a fully up-to-date but LOCAL backup drive.

A sparse bundle is only required when the backup destination is on a Server, whose native file system is unknown and frankly "don't care". Putting the entire Time Machine required "stuff" inside a sparse bundle means Time machine can ignore everything about the native file system on the Server, as long as it supports the creation of the required sparse bundle for each backup set.

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That is why I was trying to suggest earlier, lack of a second sparse bundle on your Server drive suggested that Time Machine never even really got started with backup for the second Mac.

You should be able to use Finder to navigate to the Server drive, and examine what files are there, without even mussing up you hair.

if you are lucky, you might be able to manually create the required sparse bundle file, or figure out if permission on the Server drive were stopping that from happening form the second Mac.

NB >> the login credential for the Server MUST be available on the second Mac to make the connection and write its files.

What files are present at top-level of the backup drive on the NAS, comparable to the James Pond article I cited earlier?

Do you have a sparse bundle for each of the computers being backed up?

<< I wish to back up only 300 Gb >>

Time Machine does not have a setting for 'Back up Only 300GB of my stuff'.

The way you reduce the amount to be backed up is to use the "exclude" list.

¿Do you have enough space available on the NAS data drive to hold another Mac's backup?

rossinholet wrote:

I just don't understand why Time Machine goes "skipping" without me pressing the button to "skip this backup". It's as if Time Machine was giving up and telling me that the backup cannot be completed.

That is because Time Machine is giving up and telling you the backup cannot be completed.

...and based on the elapsed time, likely not even seriously started.

You can always add an additional Time Machine destination at any time. Local and remote destinations can be mixed.

Without removing your locally-attached drive, you can add the Server as an additional Time Machine backup destination, once you get it working.

When you have more than one, every-other backup goes to every-other drive. Each is a standalone backup set and can be removed whenever you wish.