Files inexplicably don't fit on Unraid share

Hi everyone,

I'm transferring files from a 6TB drive (almost full) on a Windows PC to Unraid, which has two 6TB drives (one as parity). However, I'm having an issue where the transfer has stopped ~80% of the way through because there's no free space left on the NAS.

The Unraid disks use XFS with encryption, and currently there is no cache assigned.

If anyone could help me understand why this is happening or some potential fixes it would be much appreciated.

Thanks