AirPort Time Capsule won't back up with Time Machine

If necessary you can "reconnect" to the backup disk using the procedure described below.

The following technique can be used to "reconnect" to the backup disk. It will not affect the existing backup history in any way.

Open Time Machine's Preferences or Settings and de-select the Back Up Automatically checkbox. Make sure no backups are in progress, otherwise you should wait for it to finish or terminate.

• If the backup disk is directly connected to the Mac, disconnect it: drag its icon to the Trash, wait for it to disappear from the Desktop, and then physically disconnect it from the Mac.
• If its icon does not appear on the Desktop, select External Disks in the Finder menu > Preferences... > General. You can de-select it later.

Then, click the "Select Disk..." button, select the backup disk, then Remove Disk, then Stop Using This Disk.

• If you designated more than one backup destination, the "Select Disk..." button is replaced by an "Add or Remove Backup Disk..." option. You may have to scroll through the list of backup disks for that option to become visible.

This will not affect any existing backups.

• If the backup disk had been directly connected to your Mac, reconnect the backup disk, power it on if required, and wait for its icon to appear on the Desktop.

Then, Select Disk... again in Time Machine's Preferences. Re-select the same one under Available Disks, then Use Disk.

• If the backup is on a network (TC or AEBS), you will be prompted for the password you provided for that disk in AirPort Utility.
• If the backup was encrypted, you will also be asked for the backup encryption password.

"Waiting to complete first backup" will appear, which means it won't do anything until the next scheduled backup. Despite the implication of that message, it does not mean it needs to create a completely new backup, as if it's creating a brand new one. Your existing backups will still be available.

• If you don't want to wait that long, select Back Up Now.

A Notification may appear if your source volume is encrypted and the backup volume is not. That's normal.

Re-select Back Up Automatically. The "Preparing Backup..." status message will remain for a long time. Eventually, "Backing up xxx of yyy" will appear, but those values will not be accurate, nor will the "Estimated time remaining" in Time Machine's Preferences. Just ignore it. It may take a few hours even if your Mac is not allowed to sleep. Time Machine will also pause or become slow if you use your Mac for tasks it considers a higher priority.

Contrary to what you may have heard, the most recent Apple AirPort Time Capsule model remains fully supported despite the fact it has been discontinued for over five years.

All Time Capsules are officially obsolete, unpatched, insecure etc. etc. It’s no longer a reliable solution. Your disk could fail and/or backups go corrupt without warning or notification.

Why would anyone want to use it as back up solution today? Just say No, stow it for recycling and find another solution. Good luck!

That depends on which year/generation it is. Is it the supported 5th generation 2013 one or earlier?

Yes, any external backup disk will suffice, and of course it's always a good idea to have more than just one Time Machine backup. You can connect that hard disk drive directly to the Time Capsule's USB port or directly to the Mac it's backing up.

If you don't get anywhere you can always "hard reset" the TC: How to reset your AirPort base station - Apple Support. That will not affect its backup history, but you will need to "reconnect" to it after the "hard reset" and subsequent reconfiguration. It's generally required to perform that "hard reset" when there has been a change to your network configuration, and should not be necessary otherwise. However, it's simple enough to accomplish and should resolve the inability to find the TC.

Possibly since it is outdated it is not compatible with my 2023 Mac Mini?

It may be outdated but it remains totally compatible.

I did not ask but what is the TC LED status? On / off / green / amber / blinking? Normally it should be solid green. Flashing amber means it needs some kind of attention.

One more thing. You wrote that you use it as a secondary router. You can only have one router on a LAN at any given time, so I'm not quite sure how it's configured on your network. The "hard reset" will lead you through its configuration options. Unless the router you're using is an Apple AirPort Base Station (nearly any model) then the only way to use the TC as a backup device is to physically connect its Ethernet port to that router.

I stand corrected, this most recent one 🙄 from 2013, the 5th generation AirPort Time Capsule 802.11ac - Technical Specifications - Apple Support isn’t (yet) officially obsolete.