@anildash 

Were you on Big Sur?

@anildash FYI just got and set up an AX11000 and was getting the Time Machine problems described here. (A) thanks everyone for figuring this out. (B) I was able ot fix the problem just by reformatting the drive to Mac extended journaled, but did not have to make the bakup encrypted.

Of course, if you want encrypted...

It's such a pain.

Never had problem in over 5 years with my old router. But since it was working well I think I'll just return the Tp-link. I wanted to have more range than my old airport extreme but finally there is not so much difference.

Anyways, finally I did make my first computer connect to time machine, (mba on Big Sur),

but the second can't backup to it.

I always get the "can't backup on time machine" error message.

The second computer is a mbp on Mojave (10.14).

@wesd 

I've identified the (or at least 'a')  problem and notified TP-Link tech support. While it's on my AX6000 I am pretty sure it's across all platforms. The problem is that by default, if formatting HFS+ extended (both case and non case sensitive) it also formats journaling. The router cannot handle the journal, and overwrites it corrupting the filesystem. You don't see it until the next reboot, which is why you can do the first backup, but rebooting the router, or disconnecting the drive triggers the corruption.  To fix, disable the journal. On your mac after formatting, mount the drive and get a command line (open terminal). Assume that your drive is named NameOfYourDisk. nter the following command at the prompt:

diskutil disableJournal /Volumes/NameOfYourDisk

Eject the drive and then attach it to the router - that should do the trick. Obviously, test this yourself on something that does not have important data as formatting will wipe it.

And equally important - without the journal, a power outage or reboot while the disk is being written has a chance of causing filesystem damage that you will need to connect to the mac to fix. 

Typical disclaimer applies: I'm not a TP-Link employee, and your milage may vary, but it's working for me.

@wesd I just got an AX73 and was unable to get Time Machine to work properly/consistently...It would do the inital backup but subsequent it would hang on searching for disk.  IIn the AX73's config there are two options..one to configure the drive and one for configuring the drive for Time Machine. I tried a number of formats an nothing worked.  I had given up trying to get it to work and I went and disabled the Time Machine in the AX73's config.  Just for fun I decided to try a Time Machine back up and it worked...I didn't get too excited because I was able to get the back up to work the first time.  Then, I decided to do a back up now and that worked.  I left it alone and it has been backing up every hour as expected.  Go figure.

@wesd try disabling Time Machine in the router.

@MAO219 Have you rebooted the router since you attached the drive? That's when mine was corrupting the drive.

Just a quick note, I formatted the exteranal as ExFat and TM seems to be working as it should.  Have three macs doing encrypted back ups to the TM "Archer AX21"