Airport Time Capsule

To answer your principle question

Yes.

(1) Can both Macbooks be backed up to the same Time Capsule Disk Partition where the new one is currently backing up?

Yes, in fact there is only one available partition on the Time Capsule. It is by default named Data unless you changed it.

(2) If yes, is it okay for me to "Add or Remove Backup Disk" under Time Machine Preferences, remove the two Backup Disks that are not available, and select Use Disk option for the Backup Disk that is being indicated as Available?

Yes.. that is the correct approach. Time Machine creates a Sparsebundle. This is a virtual disk file.. so is kinda like a partition. Each backup device will create its own Sparsebundle. (also named BackupBundle in Catalina for unknown reasons).

When you do that.. Time Machine will state it is going to start a new backup.. it will then check existing backups and if it discovers a previously created one, will spend quite a bit of time checking it.. then finally doing the next incremental backup.

If it still has the same issue.. cannot find the backup.. post back and we can suggest other ways around the problem.. Please post full network details and screenshots from airport utility of the TC setup. It helps loads to locate issues.

Bob's post here is also worthwhile following if the backup proves unreliable.

https://discussions.apple.com/thread/253585694

Note. As you discovered, Apple has indeed made the TC obsolete and it is far less reliable.. than its previous point in unreliability.. which was about -8/10... so now it is -9.99/10 ..

For some it works fine.. but I never found the combo of TC and TM particularly wonderful.

LaPastenague lives in Australia, so he may not be back on the forum for a time. I have tried to answer your last post. He may have further feedback for you later.

above screenshots show status of Airport Utility, Disk Utility, and Backup Failure Error message.

Thanks, screenshots really help.

AirPort Utility looks good on the surface, but there may still be issues with the Time Capsule hard drive. We are going to get deeper into AirPort Utility later in this post.

Disk Utility........you do NOT use Disk Utility on a Time Capsule because the Time Capsule is a "network" drive. In other words, the drive is connected to your network router. Disk Utility only works on "local" hard drives that are connected directly to a Mac......like a USB drive. So, delete Disk Utility from your troubleshooting list with a Time Capsule, since it will not normally be used with a Time Capsule.

Backup Failure Message is an indication that there is a problem with the hard drive inside the Time Capsule. Here is what I would like you to do:

Open AirPort Utility and click on the picture of the Time Capsule

A smaller window will appear. Click Edit in that window

Click the Disks tab at the top of the next window

Click Erase Disk

In the next window that appears, select the Zero Out Data option

Click Erase

This is a secure erase that will take a long time.......probably 8-10 hours. The Time Capsule indicator light will blink amber during this time, but you can still use the router and wireless portions of the Time Capsule.

If you keep AirPort Utility open during the Erase process, it will indicate the status and progress of the Erase Disk action.

When the process is complete, power off the Time Capsule, wait a minute, then power it back up again

Restart the 2013 Mac and try to run a Time Machine backup

You may see "Preparing" for a long time.

That's OK. Let the backup try to run until it completes, or until you see an error message.

If a message appears, post back with the exact text or a screenshot of that message please.

If the backup of the 2013 Mac is successful, restart the M1 Mac and try to run a backup

By the way, I am shortly planning to upgrade my 2013 Macbook to a second M1, after which both my M1 and my wife's 2021 M1 is expected to run the same OS version. Is that expected to help?

Unfortunately, no. Newer operating systems like Mojave, Big Sur and Monterey are having far more Time Machine issues with a Time Capsule than Macs running older operating systems. There are reasons for this, but I would not expect Apple to address them since they discontinued the Time Capsule almost 4 years ago.

You can use the Time Capsule if it works, but it would also be a very good idea to come up with a second backup plan that does not make use of the Time Capsule. The simplest, fastest and most reliable way to back up is connect a USB hard drive directly to a Mac.

I would also recommend that you look at a second backup application option. Carbon Copy Cloner (CCC) is a very popular and reliable application to back up your Macs. It won't create a history of backups though, it simply keeps an update to date current copy of your Mac.

I have no interest in CCC other than that as a very satisfied user.

You might have meant to say that you used the Zero Out Data option to erase the Time Capsule.

This would normally take about 7-8 hours on a 2 TB Time Capsule and 11-12 hours on a 3 TB version. If you chose the 7 Pass Erase option, this would have taken 7 times longer. Puzzled about the 30 hour estimated time for completion.

Overheating message is not a good sign, but it can sometimes be triggered by a bug when there is really nothing wrong at all. When the erase is done, we can try a few things.

Hi Som

Good work on rearranging things.. and as Bob mentioned.. screenshots are really big help.

The error message is accessing the sparsebundle. That means the issue is not network but actual internal issues between the Mac and TC.

You have shown us screenshot of the TM error message and disk utility showing the opened sparsebundle. Did you have disk utility open the sparsebundle before or after the attempt at backup. If the former you will get that error message when you attempt to backup because the sparsebundle is mounted to the computer.

Our first cure for this problem is to reboot the TC.. but more exactly it is to ensure the sparsebundle is not mounted. Often TM does not dismount the sparsebundle correctly so you must reboot the TC anyway..

So first solution.. reboot the TC and try again.

Also as Bob commented disk utility can read a mounted sparsebundle but is dangerous to use for anything else. Don't use it.

Second solution is same as first but more extensive.. reboot the entire network.. power off everything.. power on your main modem router.. wait 2min.. power on the TC.. wait 2min.. power on the Mac under test. Try your backup.

If that works it maybe short lived and you will need to post back again so we can give details of how to find the TC on a network.. we also need more details of the network.

What modem router make and model?

How the TC is configured? Main router or bridged is important.. but just post screenshots.

Bob has already got you doing a disk erase. I doubt this will help and of course will delete all your current backups. It is still a worthwhile check of the hard disk status.

planning to upgrade my 2013 Macbook to a second M1, after which both my M1 and my wife's 2021 M1 is expected to run the same OS version. Is that expected to help?

No the issue is broader than OS.

have taken a separate Seagate backup anyway prior to buying a new Macbook.

Excellent. Never have too many backups.

And finally, you mentioned you dont like TC + TM combine. What do you use for automatic network based backup?

Many of us here started using Carbon Copy Cloner (CCC) as a way to preserve backups when Time Machine was constantly corrupting and wiping its old backup and starting over. Same as Bob; no pecuniary interest. We recommend it because it is a fair price and works very well with lots of info and KB. It has never corrupted and wiped out its own backup.

My current setup is an desktop Mac with 4TB external USB drive permanently connected. Split into 2 partitions, I use Time Machine in auto to one partition and CCC to the other as a bootable clone.

On the network I have TC.. A1470 plus lots of old ones.. like A1409 and even a A1254.. but I have now stopped regular Time Machine backups to them. I do use it occasionally when asked a question here I need screenshots for.. and my old laptop MBP 2011 also does occasional TM backups to it.

I also use Synology NAS. This can be configured to do either CCC or TM backups.. So I swap occasionally between them.

My main backup is the local disk since CCC allows me to do bootable backups only in this configuration. I can then choose occasionally to boot the computer from backup to ensure the backup is working perfectly.. this takes a few minutes and is a life saver since checking backups is only possible with TM by doing a full recovery which takes many hours to do.. and is not even possible since Big Sur. From Big Sur you must install a clean OS and then migrate your backup.. very time consuming. Nobody is ever going to test if the backup works under these requirements.

Since TC are no longer available change is coming.. like it or lump it.

I strongly recommend to everyone to use a local disk.

Easy peasy on a desktop. For a laptop it is a pain.. so think about ways to automate it. eg

Use a USB charging hub, and leaving the disk plugged into the hub. Whenever you plug the computer back to charge it will also do backups to local USB disk.. on the move it makes backups to main boot drive which it preserves once you plug back to charge. USB charging hubs are fairly cheap now.. check OWC for example.

For network backups buy a Synology NAS. The cheapest two bay is fine with just one large disk in it. Still expensive but will usually last several years. There are also routers that support TM to USB drive plugged into the router.. eg Synology RT2600AC or new AX coming soon.. or Asus high end routers .. HOWEVER no router supports SMB3 for Time Machine backups. They all still use AFP which Apple keep available and is still secure but at some point could be dropped. Time Machine backups are now expected to use SMB3 over network. TC does not support it.. nor can it be made to support it. SMB3 is also too demanding for routers.

Part of the problems you are experiencing is using old systems and file formats which are now deprecated.

Should I Power OFF/ON the TC?

Yes, and also restart your Mac.

I have always purchased Apple Care with any new Apple computer and never have regretted it. Sometimes you come out way ahead and sometimes not.