Accidentally overwritten a partition on Time Capsule with a Time Machine backup

your data stored on Time Capsule can't be backed up

Sure it can. To back up data on the Time Capsule, you would connect a USB drive to the USB port on the back of the Time Capsule and use Apple's "Archive" function to back up everything on the Time Capsule drive.

Or, you could manually back up data from the Time Capsule drive to the USB drive using the Finder.

If more USB drives will be required, the user can add a powered USB hub to the port on the Time Capsule to connect multiple drives.

I am pretty sure that all I did was ‘right click/new folder’ when I first got it at the highest level of the Time Capsule. Can’t remember exactly though.

Ok.. so you created a folder.. not a partition.. they are ENTIRELY different.

Time Machine will not delete your folder.. it always places the Time Machine sparsebundle in the root directory.

I would say all that has happened is your permissions got switched.

It should be possible to figure out if there are hidden files..

Use the get info to find out the size of all the visible sparsebundles and any other folders that are visible.

Then check the free space on the TC drive via the airport utility or Time Machine.. How much space is being used but is invisible to the computer?

Does that equal your photos and songs?

I still prefer you to do a backup before you do the next part.. so if something goes wrong you have some method of recovery but it is up to you.

I discovered there was a backdoor in the old airport utility to change all permissions on the Time Capsule.

See the following post.

Can't access certain folders in time capsule

The problem is you cannot load 5.6.1 airport utility into Mojave.. it does work on High Sierra although you will need to do internet search for the script.. I am not allowed to give you the method here..

So any windows computer can do it.. although I am unsure if the very latest update with win10 might not give you trouble.

Allowing full guest access to read and write could have other effects on your existing backups so be warned.. this is attempt to cure your problem that can easily introduce new ones.

Tell me if you need further help with that..

There is another way in.. but it is like finding Mordor.. the way is shut.. and only I can open the paths of the dead.

The aim is for me or my family to sit on the laptop, plug in an SD card from the camera, open Finder and drag and drop them quickly somewhere, using our home WiFi network. Any ideas? The Time Capsule is our ‘hub’ for want of a better word.

Plug a NAS into the wireless router and use that.. any NAS is a true file server.. you would use it exactly as you are using the TC now.. only instead of having permissions issues which are lost on you.. the files can always be retrieved via the gui of the NAS.. and are automatically backed up.

You can also use a desktop Mac in the network as a file server to the whole network. That is virtually the same thing.. although if you have a mix of OS then permissions issues will still come up.

"I use my 2TB Time Capsule in two ways - one as an external storage device, and the other as a Time Machine back up. "

Very poor plan, your data stored on Time Capsule can't be backed up and interfere with the Time Machine garbage collection.

If Time Machine has backed up over your data on the Time Capsule, it is now irrecoverable. Once something is overwritten, the original data is gone.

As I first stated, it is best to only use the complete disk with Time Machine. That will give you the most reliable backups with the least number of problems.

I suggest you use another disk for your data. Once you do that then you can configure Time Machine to backup that data so that it is protected.

One night it came up and I think I clicked from "available disks" the name of the partition where I save all my photos and music.

How did you partition the Time Capsule?

I am trying hard to think of what you did but nothing is really viable here. Apple provided no method of partitioning the Time Capsule..

Internet sites that talk about partitioning the TC are actually doing something horrible, like partitioning the Time Machine sparsebundle since it is a virtual disk. That will totally ruin the original backup as well as having who knows what effect on your files.. I doubt it would delete them.. so your files could still be there. If you are running High Sierra or Mojave on the computer it could have changed permissions and so hidden your files from the computer.

I would immediately plug a 2TB USB disk or larger into the TC and archive it.. whatever horrible mess you made.. at least you then have a copy of it.. and you can then plug that disk directly into the computer and mount each sparse bundle and search inside it for your files.. you can also work on the TC to fix its permissions.. which maybe the reason you cannot find your files now.

What OS are you running on the computer?

Into the future you MUST not use the TC as a NAS. It is not a file server and Apple never intended you to use it as such.. and although you can backup using a USB drive plugged into the TC.. it is terribly slow. You need either a local disk plugged directly into the Mac.. or a real NAS with proper backup configuration.

Ideally, you would want a dedicated Network Attached Storage (NAS) device. These can be attached directly to your home's Internet router by Ethernet ... but can be accessed by either wired or wireless clients on the local network.

FWIW, I use a Synology DS916 for both file storage and for Time Machine backups. I can also backup my Windows-based PCs to it as well.

Retrieve what data?

The data overwritten by Time Machine is gone and there is nothing there to retrieve now.