Time Machine will not complete backup on macOS Catalina

I too had a similar problem after upgrading to Catalina from Mojave.  My four-year-old network attached storage (NAS) back-up system (WD My Cloud EX4 in RAID 5) set-up got weird on me too.

Everything's fine now, though.

So, what did it take to resolve my issues?  I re-booted my router.  That's all.  Too, simple?

Since the NAS supported "AFP", I was already using "afp://(IP address of the EX4)" in its configuration.  But I still had problems creating a persistent connection in Catalina that would even survive a system re-boot!  It took me a while before I even knew this was a problem because I have always just let the iMac go to "sleep", which prevented it from getting re-booted! 

With a re-booted iMac, I finally started paying attention to an error message from Catalina that suggested it couldn't establish a connection in Finder because it couldn't find the default NAS user ("admin").  Since I was connecting to the NAS operating system "dashboard", via HTML, I began to believe something was now different with how Catalina was processing "data-transport protocol" connections (ports maybe?).

After bypassing my switches with no improvement, this left the router as a prime suspect.  I re-booted it and bingo, I could now create a persistent connection in Finder to the NAS user "admin" (or, any other user).

To recap:

1. re-boot the router 
2. re-boot the iMac (I now do this before I setup any network connection!)
3. re-establish the drive unit's path in Finder
4. used the "Connect to Server" app with the same server address (as always)
5. make sure it creates the "eject" icons at each level in Finder

Give it a try.  It may not address your issue(s), but at least it's not a lengthy, complex procedure and it will clear the decks!

Good luck and best regards,

OM2  

Prior to Catalina, macOS has never supported Time Machine to a NAS, despite the NAS claiming it did.

One change they made was a specification for Time Machine over SMB. If your NAS supports that Specification, then it should work. My guess is it doesn't.

Time Machine used to require AFP to work. Apple never licensed AFP, so the NAS manufacturers used a hack of AFP that didn't fully support AFP, especially the Time Machine requirements. While people generally had successful backups, restoring often failed.

Since QNAP claimed they had a device which supports Time Machine, ask them for help.

Apple’s own “NAS” for backups. Guess what. Same issue...

Maybe it is something with your network.

I don't have any problem backing up over network to an external drive hosted from a Mini running Catalina.

When I first installed, I had to stop and restart File Sharing, but no issues since then.

Congratulations.  Serendipity strikes again!  

I'm assuming QNAP has a default user name.  My WD EX4 uses "admin".  If not specified, it will connect to "admin" every time.  It can be changed, of course.

Did your original Mojave install use the same, or similar, NAS name for the upgrade as your clean install of Catalina?  Again, just curious.

In my pre-retirement work life (centered on very large, enterprise-level operating systems), I always had the benefit of a "release document" for major, or even "point", release activity.  This was extremely useful to help understand exactly what, and how, it was changed.  Conjuring-up temporary work-arounds would be near impossible without that documentation.  I guess it's too much for the "personal computing" industry to provide us anything similar.

Apple changed Catalina Time Machine, and many other areas of the system applications as well.  My gut feeling is they tightened-up interface requirements associated with "external" devices for security purposes.  For instance, I could run Catalina Time Machine successfully to an external, USB back-up device.  My IP-networked NAS, on the other hand, was just flakey.  One minute it wouldn't connect, then five minutes later, it would but wouldn't complete an initial TM back-up!  

I had a ticket at Apple about this problem.  The tier one support tech had me perform both SMC and NVRAM resets, which didn't improve things.  She then wanted me perform a fresh install of Catalina.  I agreed to do it but wasn't ready since I didn't have a TM backup ready.  We hung up.  I was subsequently able to connect a USB drive unit and complete a full Time Machine backup. This fact made me more curious about transport interfaces.  After rebooting switches and router, I finally had a reliable, consistent connection to the NAS that Time Machine liked!

I called Apple support back to inform them what I did to fix my problem.  I told them I thought it might be something else they could add to their resolution tool-kit.  I was then told that this fix was already known and that the original support tech should have recommended I try it before the system refresh procedure.  And then finally, I was told the original support tech would be contacted and informed/reminded of "router-reboot".

Go figure.

Best regards,

OM2

Can you access the NAS operating system dashboard?

Or ping the NAS address?

OM2