Yosemite Time Machine Backup Failed Due to Full Disk
This somewhat of a long shot, but it sounds a little like you may have changed something on your disk. &quot;Invisible&quot; actions like changing file permissions or HFS+ compressing files will cause TimeMachine (TM) to assume that the files have changed and need to be backed up. Another scenario would be that the UUID of your hard disk changed. This article lists some further possibilities. So to answer your questions:

the warning is there because you would loose all previous backups and you would be left with only this one backup.
Time Machine thinks that all files have changed. My hypothesis is that you did some action changing attributes of all files or your disk's UUID.
you could try to inherit the files

I suggest you try the following:

Compare your disk's UUID with the one used in your Time Machine:

diskutil list and
look for a hidden file called com.apple.backupd.SnapshotVolumeUUID on your TM disk (not too sure about the file name, because I use sparsebundles)


Full Reset of Time Machine: Read this article for details. Basically you will have to delete /Library/Preferences/com.apple.TimeMachine.plist
Reconnect your backup. Read this article thoroughly. You will have to use either of these commands:

sudo tmutil inheritbackup /Volumes/myTM/Backups.backupdb/myMac or
sudo tmutil associatedisk -a / /Volumes/myTM/Backups.backupdb/myMac/myDisk



Note that you might loose your entire backup. Please make a copy of your Time Machine backup.