Time Machine won't back up to Time Capsule.

It's about 6 years old

That was the change over year.. so is it tall one.. looks like this.

Or the previous model which is small pizza box shaped.

BTW the tall one is Gen5 and the previous one is Gen4. I presume you have Gen5 because it is totally lacking in model number.

It's going through the process of "preparing for backup", but then tells me I don't have enough room, which is not the case.

How much free space do you have?

It will show you in Time Machine.

How big is the backup supposed to be?

How much data total do you have on the drive/s Time Machine is backing up?

Time Machine may need to delete the existing backup and start over.. this can exceed the free space.

Do other computers use the TC for backups? Or are you storing files on it?

Let me suggest a few ways ahead.

• Verify the backup.

Hold down the Option key and select the Time Machine icon.. select verify and it should then check the current backup.

• Wipe the TC drive.

Depending on the results of the verify, formatting the drive of the TC is probably the best fix.

If you wish to keep the old backup, use archive in the Disk tab. You will need a Mac formatted USB drive plugged into the TC of equal capacity or greater to the data/disk capacity.. ie 2TB or 3TB.

Then use erase. Since the TC is now 6 years old I would do a full erase. This is zero out data. It will run a low level format of the disk which ensures all the sectors are readable. This will take a few hours.

Once completed do a backup.. it will generally work on a clean disk.

• Replace Time Machine.

TM is becoming less and less reliable with each version of Mac OS .. especially since Sierra where the rot really set in.

I strongly recommend an alternative like Carbon Copy Cloner.

If you want to run TM use it to a local disk plugged directly to your computer.

BTW 6 years is a long long time for Time Capsule to survive. It is actually time to look at replacement. Apple of course no longer make them so it will mean buying other brands. Please ask if you consider this for recommendations of what to buy.

Before you go for a new backup unit.. I recommend you try out a different backup software.

You can download and use Carbon Copy Cloner for a month free.

Using this software you can backup to a sparsebundle on the TC and it will give you a good idea of what is actually failing.

I recommend you do another erase of the TC.. but just use the quick erase which takes just a couple of minutes. That will clear the old backup attempt so you can start fresh again.

Longer term I do think you are better buying a USB drive and plug it into the iMac.. it can be hidden in behind the computer and will be a lot faster (USB3 assuming you are on a 2012 or later Mac) and a lot more reliable. If Time Machine also fails on this target then it is possible the disk in the computer needs replacement or at least reformating and a clean install. Again I would test with an alternative like Carbon Copy Cloner, which does a bootable clone to a local drive which is very useful into the future should something go wrong.

If you want network backup then you need to decide between a hard disk plugged into a router. This is the cheapest alternative. You need to buy a suitable router. The one I would recommend is Synology RT2600AC.

Or you can buy a NAS.. this is expensive.. and again I would recommend the Synology brand. They seem to have done the best with handling later Mac OS which works much better with SMB2 or SMB3. Not many NAS firmware conform to Apple SMB requirements.

I would not jump into an expensive purchase though before determining if the problem is not just Time Machine on the computer or issues with the Mac internal disk.

I still feel like I should get another backup, one that is connected directly to my Mac. Do you have a recommendation for this method?

LaPastenague already noted above that......"Longer term I do think you are better buying a USB drive and plug it into the iMac.. it can be hidden in behind the computer and will be a lot faster (USB3 assuming you are on a 2012 or later Mac) and a lot more reliable."

Backing up this way.....if you want to use Time Machine, or any other backup software for that matter......would be the fastest and most reliable method that you could choose.

Personally, I have used Lacie hard drives for many years with flawless results, but there are any number of excellent choices available.

I still feel like I should get another backup, one that is connected directly to my Mac. Do you have a recommendation for this method?

I am using a HGST Touro Deskpro. 4TB

I find this brand very good.. HGST make some of the best hard disks. (It might be sold under Hitachi label).

There are lots and lots of brands out there.

However.. I do not recommend drives larger than 4TB unless you are certain they are not backup drives.

Seagate brought out a range of large Archive drives.. between 5-8TB (might even be larger now).

These use SMR read and write instead of more normal method.. and it can get super slow.

https://www.reddit.com/r/DataHoarder/comments/7kthi6/seagate_backup_plus_hub_8tb_slow_15mbs_write_speed/

Even the LaCie do use these drives in the larger sizes.. >4TB although drives tend to change from one batch to the next depending on what is available.

You can also buy a USB enclosure from places like OWC and buy your own disk.. this is come out more expensive but you do get standard or enterprise drives and not cheap SMR archive drives.

Although it is not well known.. most 2.5" drives of capacity over 2TB are also SMR drives.. and really should be avoided.

For best reliability you are better buying good quality desktop 3.5" drives.. which means they do need a separate power supply. I have partitioned the drive and use CCC to do a bootable clone on the first partition and TM on the second partition. This gives me reliable backup on both.

2.5" drives are more convenient in being self-powered and extremely small. They are a bit slower but in 1-2TB sizes are really very cheap. Do not expect the same reliability out of these drives.

Which exact model is your Time Capsule (TC)? By deleting the disk do you mean you completely erased it or just removed, then re-added it back via the Time Machine interface? Was this always an issue or just started happening recently?

Not sure how to explain this, but the Time Capsule is not connected to my machine, I access it wirelessly. So I don't see where I would access the window to erase the disk.

The fact that you connect by wireless is not relevant.. at least as far as control.

You access the controls via Airport Utility. So just start Airport Utility (from Applications/Utilities folder)

Click the icon of the TC which is hopefully working. (if it doesn't you have other issues as well).

And then click edit in the summary.

Then click on the disk tab.

Should I plug it directly into my computer instead?

When you do the initial backup a direct connection by ethernet can make a big difference in speed and give you the best possible start to a backup. Wireless is never as fast or reliable.

If you are backing up your iMac.. I would plug it into the TC into the computer if that is feasible without major network disruption. And run the backup overnight.. it should be around 10 hours for a 400GB backup. For this to be successful when the TC is no longer in the network you would need to reconfigure it as a router.. and ignore errors.

EDIT.. if you wanted to try and just continue the current backup.. you could also try Time Machine again. With the TC plugged directly into the computer it could work. (stranger things have happened).

What role does the TC play in your network now?

Is it main router or is it bridged to another router?

I think a full factory reset of the TC could also be of value. Whether you plug it into the computer or not. But certainly if you do use a direct ethernet connection.

And use all short names, no spaces and pure alphanumerics. Not Apple recommended names in the wizard as they are based on earlier protocols.

Passwords same rules but can be longer.

If the TC does give you continuing trouble after doing the reset and wipe, I would certainly look at replacement in the not too distant future.

If you need more details just tell us what you are trying to do.

Who is your ISP?

What modem / router do you have?