Windows Backup 2012R2: there is not enough space on the disk where is a lot of space

I am backing up about 260GB of data to a 1TB disk. 

For 2 weeks Backup has been reporting a error " there is not enough space" at the end of the backup proces.

This 1TB disk is completly empty. I also tried to send a backup over the network to QNAP but there is the some error.

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Robert Stuczynski (Noise) - VirtualStudy.pl: http://VirtualStudy.pro