Timemachine backup has become invisible after starting to use Airport Extreme

Not sure if this information will help solve your issue, or make it more complicated.

Time Machine backs up differently to a hard drive that is connected directly to a Mac compared to a "network" drive that is connected to a router, like a Time Capsule or AirPort Extreme.

Backups to a drive connected directly to a Mac go to into a normal folder called Backups.backupdb that is created when you run the first backup.

Backups to a network drive connected to a router go into a special container called a sparsebundle file. For technical reasons....not all of which I understand....the sparsebundle approach is more reliable over a network.

Unfortunately, you cannot start backups to Backup.backupdb and then move the drive to a network location and continue to back up to the same Time Machine backup file. Time Machine will want to start a whole new backup all over again for the sparsebundle file and then move forward with incremental backups.

Finally, unless something has changed very recently on the AirPort Extreme, the USB port on that device will not support an encrypted drive.

Yet, when you start a new Time Machine backup to a new sparsebundle file, you do have the option to encrypt the Time Machine backup over a network. This is different than encrypting the external drive itself.

Not sure if you were aware or not.....but the USB port on the AirPort Extreme (and Time Capsule) is old and slow USB 2.0. Further, the processor inside the router limits speeds to about half of normal USB 2.0 on a computer. So, backups....or anything else....going to a drive at the USB port of an AirPort router are going to be S--L--O--W.

So, it would help if you could confirm on whether you originally made Time Machine backups to a drive connected to your Mac and you are now trying to use the drive at a network location on the AirPort Extreme.

Might also help to know if you encrypted the external hard drive itself.....or whether you encrypted the Time Machine backup file...when you originally started the Time Machine backups.

I think your problem is permissions.

There are reports of people having issues after using a USB drive plugged into another computer or network device like the airports.

Apple claims to have fixed all the permissions issues.. and therefore removed the repair permissions function from disk utility.. IMHO that was sadly premature. There are still loads of permissions issues.. and now you have to fix them via command line.

Sorry I don't have a nice command line simple solution for you. You will need to google it for the OS you run. (Sierra or High Sierra).

https://gist.github.com/bzerangue/dca8fc2d63309ba2bd9f

https://eclecticlight.co/2017/06/15/something-odd-you-cant-fix-sierra-re-introdu ces-repairing-permissions/

Alternative is using Finder to change the permissions on the partition.

Resolve issues caused by changing the permissions of items in your home folder - Apple Support

I have not had much luck with this. YMMV

There were major changes at Sierra.. as a way to protect the integrity of the OS.

How do I repair permissions in MacOS High Sierra?

There is a permissions fix in the old airport utility where you can add guest access. I really have no idea if you will get this working on the USB drive plugged into the airport.. It does work on the internal drive and I have got other people here to test it.

I guess your first problem is to run 5.6.1 airport utility.. you need older Mac OS.. early Lion.. or special script from internet I cannot post URL but just google for it.. still works for me even on High Sierra.. although it was not so straight forward. Any windows computer you can use.. or bootcamp partition on a Mac.

If you go to the Disks tab, File Sharing.. by default Airport Disk Guest Access is Not Allowed.. change it to Read and Write.. and click update. This may have multiple affects I cannot predict. But it does offer one easy way out of your problem.. IF IT WORKS!!

The initial backups were to a WD mypassport for mac via USB3.0

So, the WD drive was connected directly to your Mac for this, correct?

Then I added the backups to the Airport Extreme. The backup was via Wifi and not USB. When initialising the backup to the airport extreme I chose to encrypt it and gave it a password.

Sorry, I don't understand this.

Do you have an AirPort Extreme (which is router only, with a USB port).....or....did you really mean to say that you have a Time Capsule (which is a router with a built-in hard drive)?

If you have an AirPort Extreme, the only way that you could have backed up to the WD drive would have been through the USB port on the AirPort Extreme, although you might have used wireless to connect your Mac to the AirPort Extreme.

Or, do you really have a Time Capsule, and you have connected the WD drive to the USB port on the Time Capsule?

Unfortunately, I do not have a WD drive here to test. Perhaps another user will be able to help us out.

But, I have not had any trouble at all with backups to a Lacie drive using the drive connected directly to a Mac or to an AirPort router......except that things are really slow when the drive is connected to the AirPort.

FWIW, we have seen a number of reports from users with issues connecting a USB 3.0 drive to the 2.0 port on the AirPort Extreme and Time Capsule, so not sure if this might be part of the problem.

Yes, USB 3.0 is supposed to be backwards compatible with USB 2.0, but I'm not convinced that this is always true. Keep in mind that the Time Capsule was designed at least 6 years ago, and Apple has not made any changes to the hardware since the product was introduced.

Hopefully another user with a WD 3.0 drive will be able to help us out.

There is no issue whatsoever with the time capsule. Backing up to it via wifi works perfectly.

Understood.

The problem is with the external drive and the time machine partition on it.

That is why I said that.....Hopefully another user with a WD 3.0 drive will be able to help us out.

I suspect that you have a WD USB 3.0 / Time Capsule USB 2.0 compatibility issue. The USB port on the Time Capsule is ancient. But, it works OK with my Lacie 3.0 drive on encrypted Time Machine backups.....if I don't mind waiting a long time for things to load.