Raid 6 file transfer

It sounds as if something went wrong and the volume is in read-only mode.  You should check the log in the GUI, and possibly some in the download package.  RAID6 alone will not cause that.

Perhaps I'm reading too much into this and you have backup.  But if you believe RAID6 is the best way to protect your files, you are mistaken.  Most of the same things that can destroy a RAID5 volume can do so for RAID6, and the NAS remains just as susceptible to hardware failure, theft, fire, etc.  RAID's primary purpose is availability, not backup.  If you lose a drive, you don't lose access to your files while you restore them from backup.  But for real protection, you need real backup, which means on another devcice, such as a USB drive.  For the fire and theft vulnerabilities, that backup would need to be off-site.