Time Machine couldn't complete backup I keep getting this message:

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I understood that when T M filled the target disc, it would delete the oldest backups to make space for later backups. This is fine with me, and I would be quite happy if it just did that. But it keeps warning me with this screen. I can not find a way to stop this so I have to click OK very time, sometime 20  times a day. 

I have a 1T HDD with 300Gb free and a 1T backup disc. 

Is there a way to stop this warning appearing?
When Time Machine begins a backup, it first 'prepares' the backup. It's at this point that the space requirements are calculated. If there's not enough space, it will delete old backups, and tell you so, including the date of the last backup that's left on the drive.

However, your error is that it's run out of space unexpectedly. This is much more rare.


Try repairing your drive with Disk Utility

There might be a problem with the disk and/or partition, where directories are smaller than they might first appear. Try repairing it.
Another process is writing to the drive

Make sure that nothing else is being saved to the partition at the same time. If another process writes a big file to the partition, then the space requirements won't match, and you'll get this error.