Securely wipe old time capsule

On your Mac, open up AirPort Utility

Click on the older Time Capsule, then click Edit in the smaller window that appears

Click the Disks tab at the top of the next screen

Click Erase

On the next window that appears.....

Select the Zero Out erase option

Click Erase Disk

This is the fastest secure erase setting, which will replace everything on the drive with zeros. It will take 3-5 hours on average for the operation to complete.

If you want an even more secure erase, choose the 7 Pass option. This will Zero Out the drive 7 times, and as you might expect, it will take 7 times longer for this to complete.

There is a 35 Pass option, but this will take weeks to complete.

Once the erase operation is complete, you can return the Time Capsule to its original factory default settings as follows:

Open AirPort Utility again, click on the Time Capsule, then click Edit in the smaller window that appears

Click the Base Station tab at the uppermost top of the screen, up where you see the Apple icon, and other menus for File, Window, Edit, Help, etc.

Click Restore Default Settings

Click on the image below if you need to enlarge the display.

The problem here is that the Time Capsule has already been reset back to factory default settings, so it is not possible to access the settings of the Time Capsule until it has been set up to work on your network.

We can try a workaround to see if that will get into the Time Capsule without having to set it up again, but your Mac will need to have an Ethernet port and you will need a short, spare Ethernet cable.....any length will do. Do you have these items?

Even though you might have reset the old Time Capsule back to factory default settings, this only changed the network settings. All of the data on the Time Capsule hard disk remains unchanged.

This will seem a bit strange to you, but please follow these instructions to provide AirPort Utility with the best chance to be able to get into the settings of the old Time Capsule.

Turn off the wireless on the Mac

Connect an Ethernet cable from the Mac to one of the three LAN <--> ports on the old Time Capsule

Open AirPort Utility

Click on the File menu in the upper left corner of the screen and click on Configure Other

Enter the following in the Address box

10.0.1.1

Leave the password box blank for now

Click OK

If AirPort Utility asks for a password, then enter.....

public

See if that will result in a display that shows a number of tabs across the top of the window.......

Summary, Base Station, Internet, Wireless, Network, and Disks

Click on the Disk tab and follow the instructions that I posted above

Since your Mac will be out of commission during this time, you might want to perform the erase procedure late at night, right before you go to bed so the erase will occur overnight

So with the WiFi off on my iMac, and the ethernet connected direct to the TC, do I have to leave it like this when its erasing?

Yes, please note that I said above that your Mac will be out of commission until the erase operation is complete.

but what happens when my iMac goes to sleep, as it does after 15mins, will it continue to wipe?

I've never changed any of the settings on the Mac when I've done this in the past. But, it would not hurt to tell the Mac to "never sleep" until the erase operation is complete. Then, change the settings back to normal once the drive has been erased.