I have a 3TB time capsule that's c. 2 years old. It worked perfectly with my old iMac. I bought a new 27" one when it came out this year and since the time capsule will not work.
I had to clear all the data as a starter. Now it gets a little into the backup taking c. 1 day then resets. It's connected via ethernet cable and has been told to join an existing network.
Currently it says it's backing up 922,498 items, after 2 hours (this is the third attempt, it resets then has no record of the old backup) it says 10kb of 789.44 GB - About 4,664,440 days remaining!!
To clarify I'm trying to use Time Machine with the Time Capsule.
You have made a mistake in the setup.
You do not "Join an existing networking" and use ethernet.. they are mutually exclusive.
What you are doing is backing up over double hop wireless which is slowing to a crawl.
You need to put the TC into bridge mode.. and plug it into the main router by ethernet. Then use wiretess to the TC if you are too far away or ethernet if you are close by..
As soon as you put the TC into join a wireless network it deactivates the ethernet.