DLINK DNS 323 NAS out of space error message

I bought a Dlink NAS and made a RAID 1 with 2 WD 750gbs drives. The first time I tried to copy all my files it gave me an out of space error message. The size of the files being copied were close to 80gbs. Being a RAID 1, I have available 750gbs. I don’t know why it gave me that message after having all this space. One of the folders I tried to copy included an image of my PC, total 54gbs.

Any ideas?