Time Capsule Fails Backing Up (even after months of it working)

Question

Ive run across this numerous times over different macs (24 inch, 27 inch, air...) on every official os supported (snow leopard, lion)
I set up time capsule initially and it works great.
After what could be months, it will error out (backup failed).
It finds the drive and manually initiating a backup fails as well.
Funny things is it says something to the effect of "backup requires 9.gb, Only 3gb available" or something...
I thought this thing deleted old backups as needed?
Why does this always happen given time to allow it to happen?
I have 2tb TC with only 1 TB hd on the current mac.... double the space yet given enough time it will always happen.
The only way to get it to start doing what its supposed to do is format and start over... and Im really over doing that.
I have excluded all the folders I can. If I keep excluding folders so that it will backup, whats the point?
Does anyone have answers or knowledge on the following:
1. Why would it stop working over time? Even if I add a large file to the mac, shouldnt it "make room" for the new file by deleting old backups?
2. How can the backups equal more than the entire HD? I understand it takes snapshots but if a file is deleted off the mac, shouldnt that reflect in the backup over time and be deleted as well?
3. Most importantly, how can I get this to start working properly again without reformatting and starting over or excluding yet another folder?
Thank you so much for any assistance anyone may have on this.

Answer 1

reifel tower wrote:
 2. How can the backups equal more than the entire HD? I understand it takes snapshots but if a file is deleted off the mac, shouldnt that reflect in the backup over time and be deleted as well? 
3. Most importantly, how can I get this to start working properly again without reformatting and starting over or excluding yet another folder?
Thank you so much for any assistance anyone may have on this.
Perhaps I can help with a misunderstanding on one point and others more knowledgable can jump in on some other points.
The snapshot always saves whatever is different. That is the whole idea, that files you are adding to the hdd in the Mac are copied to the backup. They are not deleted from the backup because you deleted it from the Mac.. the idea of a snapshot is to preserve an accurate record of all files at that point. Time Machine is designed to help when you accidentally erase a file.. or make a mistake.. or a file is corrupted.. so you need to go back to an earlier revision of the file. So by design TM is working as incremental backup device.
If you have a large amount of your 1TB full, and you are shifting files, of 3 or 9GB which you list in the problem description, and those files are being moved around often.. even changing their names. The whole file will be backed up every time Time Machine runs. Eventually it runs out of space.
3. To answer why it doesn't just delete the older backups I am really unsure. That is the design of the software, and it should delete older backups to make room for new ones. Let me suggest a change of strategy based on the above. Files that you are constantly moving around put them into an excluded directory, assuming you don't need them backed up. Make sure you reserve the TC for TM and don't put other stuff on the disk.. perhaps using too much space for copied files.. if you do that.. the TM cannot remove those files and runs out of a small enough backup to delete without affecting something else.