Airport not backing up wirelessly

The last time I backed up to the airport (using ethernet) it gave me an error message, that I can't quite remember, but something like "we have to repair your backup, which requires deleting your old backup files" -- only after backing up to USB hard drive then going back to the airport did I try this. Still only a sparse disk image bundle (not folder) and still not working wirelessly.

This sounds as if you saw this dreaded message:

The message is somewhat vague, but it is telling you that your current sparsebundle file of backups on the Time Capsule is corrupted, so backups will not occur again until you click to "Start New Backup".

When you do this, Time Machine will first erase all of the old backups of your Mac, then start all over again with a new backup.

If you click Back Up Later, nothing will happen, and you will have to click Start New Backup later.

From your message though.....I am not clear on whether you clicked to Start New Backup or not.

Can you clarify on that please?

I go to Finder to make sure the airport is accessible, and it takes a long time (30 sec) to connect

This is normal in my experience with later versions of Sierra and all versions of High Sierra.

Sometimes it even asks me to enter the airport password, even though I always have the box checked for "Remember this password in my keychain."

If your Mac is running High Sierra, you will now normally be asked to enter the password every time that you access the Time Capsule using the Finder. Depending on what version of Sierra that you are using.....(if you still have that installed on your Mac).....you will likely be asked to enter the password when you access the Time Capsule using the Finder.

This is new increased "security" that Apple has added to High Sierra and later versions of Sierra. I know of no way to override this new security "feature", but one of the gurus who understand the ins and outs of Terminal might be able to come up with a command that would allow you to access the Time Capsule without having to enter the password each time. Please start a new post if you want to pursue that question.

Unfortunately, you are one of countless numbers of users.....including me.....that have experienced intermittent Time Machine problems since Sierra and High Sierra have been introduced.

What may help....or may not....is to manually perform a Zero Out Erase procedure on the Time Capsule hard drive to completely wipe the drive and then start over again with a new backup. Post back if you want to try this and are not sure of the procedure. Again though.....there are no guarantees that this will help with the current Time Machine issues.

When you click "Start New Backup" in the error message box that you see above, Time Machine performs what is known as a "Quick Erase", which might not be completely erasing the errors that are present on the hard drive. That is an educated guess on my part.

Frankly, I no longer use Time Machine and a Time Capsule for my primary backups. Instead, I have switched to USB drives that connect directly to each Mac and use Time Machine that way. So far.....I have had no issues over the last year or two backing up this way.

During the same timeframe, I've had at least 3 or 4 corruption problems with Time Machine backups to my Time Capsule, which force me to delete the old backups and start all over again with a new backup each time the problem occurs.

Also, it's still saving the backup as a sparse disk image bundle

Time Machine has always backed up to a sparsebundle file with the Time Capsule, since it is a network drive. Time Machine will continue to backup to a sparsebundle file. You cannot change this.

Time Machine will automatically back up to a normal folder called Backups.backupdb when you are using a USB drive or FireWire drive that is connected directly to a Mac.

The bottom line.....I have not been able to come up with a reliable fix for Time Machine backups to a Time Capsule. My advice would be to limit your use of the Time Capsule as much as possible, and rely on another backup plan for your important data.

Even if you can get things working with Time Machine and the Time Capsule, it will not be long before you see another error message on your Mac's screen.....my opinion.

Let "Preparing" run until the backup completes.....or....you receive an error message. It is normal for "Preparing" to take a long time after you have made any major changes to the Time Capsule.

"Preparing" ran for over 10 hours recently when I had the same kind of corruption issue, and the new backup was over a wired Ethernet connection (would have been even longer over the slower wireless)......so allow a day or two to see if things will straighten themselves out.

Sierra (10.12.x) and High Sierra (10.13.x) have had numerous problems with Time Machine sparsebundle backups becoming corrupted.

So, it is likely that you will need to repeat the process again. The question is "when"?

No reason to stop using the Time Capsule. My advice was to not think of the Time Capsule as your primary backup plan.

I still back up to a Time Capsule....in the hope that it will be working if one of my other external hard drives fails.