Back up full?

Question

I've been using a WD 2TB backup disk via UBS or a long time and it's worked like a charm with Time Machine.
I got a Time Capsule the other week cause I got fiber internet installed in my house and figured it would be good to have all Apple and then use it as an extra backup disc as well. That of course goes wireless.
I got another external disc with all my tv shows, movies and pictures that is always hooked up to my computer when I back up. That is about 850GB. My computer is pretty empty, I'm using like 125GB.
So.. for the longest time my WD have been backing up around 1.1TB without any problems and I've had backups going back about a year. No problems what so ever.
I set up my Time Capsule a couple of weeks ago, takes about a day to back everything up into that but when that is done, all is good. I now have backups on my WD and Time Capsule.
A few days in, I realize only Time Capsule is doing backups. I didn't do anything about it cause I knew I had old backups in WD and the new backups was in Time Capsule. I was going to do something but just never got around to it.
Yesterday I get a message that Time Machine could not backup to Time Capsule cause it was full. About 850GB was available but it needed 1.1TB to backup. My first question is, Time Machine doesn't need the full backup space EVERY time it backs up right? My WD certainly didn't since I had a years worth of backups in the space of 2TB.
So I try to google yesterday to find a solution for this but it was late and I was tired. So I since Time Capsule didn't want to backup, the WD showed me that is was interested in backing up again (I guess when one disk won't backup the other one gets more interested..). So I let the WD do a backup and went to bed. This morning when I woke I see a message saying the WD is full and won't be able to do any more backups cause it needs 1.1TB and only about 850 is available. Same message as Time Capsule gave me. That has never happened before with WD.
So I go into Time Machine to check it out and now the oldest backup it has is from yesterday!! All last years backups is gone.
How come all of a sudden I need so much more space to do backups? It's been working fine for years. And it's not like it's more that needs to be in the backup. It's been the same amount (about 1,1-1,3TB).
Does anyone has a solution on how I can continue to use both disks as backup without having to get this error message?
Thanks so much
Sophie

Answer 1

This procedure is a diagnostic test. It makes no changes to your data.
Please triple-click anywhere in the line below on this page to select it:
tmutil compare | open -ef
Copy the selected text to the Clipboard by pressing the key combination command-C.
Launch the built-in Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad and start typing the name.
Paste into the Terminal window by pressing command-V. I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key after pasting.
The command may take several minutes to run. Wait for a new line ending in a dollar sign ($) to appear below what you entered.
A TextEdit window will open with the output of the command. Don't post all the output; just read it yourself. Each line that begins with a plus sign (“+”) represents a file that has been added to the source volume since the last snapshot was taken. These files have not been backed up yet.
Each line that begins with an exclamation point (“!”) represents a file that has changed on the source volume. These files have been backed up, but not in their present state.
Each line that begins with a minus sign (“-“) represents a file that has been removed from the source volume.
Files that you’ve excluded from backup, or that are excluded automatically, are ignored.
At the end of the output, you’ll get some lines like the following:
Added:
Removed:
Changed:
These lines show the total amount of data added, removed, or changed on the source(s) since the last snapshot.
Caution: Under some conditions, tmutil may falsely flag files as not having been backed up when in fact they have been. It can be confused by multi-linked files, or by files with a modification date earlier than the one in the snapshot. This will not be an issue for most users. If the results of the above procedure surprise you, inspect your snapshots directly.