wireless backup to airport extreme

Unfortunately, the message that you are seeing is almost always an indication that your Time Machine backup file has become corrupted and is no longer useable.

But, this message does not usually appear unless you already have been backing up to the Time Capsule in the past. Your post seems to read as if you are trying to back up the Mac to the Time Capsule for the first time.

Can you clarify on whether the Mac has been backing up in the past and is now having difficult, or whether you are trying to back up the Mac for the first time?

We assume that you have already performed basic troubleshooting steps like shutting down your Mac, powering off the Time Capsule for a few minutes, powering the Time Capsule back up and then starting up your Mac again. Would that be correct?

The next troubleshooting step would be to try to verify the integrity of your backups. If you have not already done this.......

Hold down the option key on your Mac while you click on the Time Machine "clock" icon at the top of the Mac's screen

Click on Verify Backups

If the verification process can run, it might take anywhere from a few minutes to an hour or more to check the Time Capsule hard drive depending on how much data is stored on the Time Capsule.

Post back on the results of the Verify Backups test.

Is the TC bridged to another router?

If so there are couple of things you can do to improve the reliability.

First .. set the TC to a static IP.. this is easy to do .. just change from DHCP to static on the Internet tab.

Make sure the DNS is set to something public like Google.. 8.8.8.8

Second.. make sure you are using IPv6 set to link-local only.. even if your main router is set to IPv6 and the computer is using IPv6.. The TC does not like using both IPv6 and IPv4.. at the same time.

Post back if you have more issues.. after a week or two.. as TM is less reliable with each Mac OS release.

An Extreme does not have inbuilt hard disk.. only the Time Capsule has that.

So for Extreme you can use a Mac HFS+ format hard disk plugged into the Extreme. Is this what you have?

What model is the Extreme as this setup was never particularly reliable until the Gen6 tower model Extreme?

What OS are you running on the computer?

Have you mounted the disk in Finder and actually copied a file to it. This is well worth testing.

What error messages are you getting in Time Machine?

You can try things a few more times, but everything that you are telling us indicates that your backup file is corrupted and no longer useable.

The simplest thing to do is erase the Time Capsule drive and start over again. But, if other Macs have been backing up to the Time Capsule and you don't want to lose those backups, then you can usually delete only the backup file for the Mac that is having problems. Things are more complicated this way.

Unfortunately, Time Machine has not been all that reliable backing up to a Time Capsule since High Sierra was introduced, so starting over is not likely going to be a permanent solution. More than likely, the backups will become corrupted again. The only question here is when......not if.

Time Machine is a lot more reliable if you can connect a USB drive directly to your Mac and back up that way in addition to the Time Capsule.

Backup suddenly started working after the verification.

Let's hope that things will behave going forward.

About once a month, it would be a very good idea for you to........

Hold down the Shift key while you click on the Time Machine "clock" icon at the top of the Mac's screen

Click on Back Up With Consistency Scan