Moving Hyper-V virtual disk to SMB share gives file not found error in the end

The NAS is a 2U Synology Rack set up in SMB 2.0 and higher mode. Plenty of space on the share (and NAS). The server is Windows 2012 R2

SMB 2.xx is your problem. You need SMB 3.xx to allow Hyper-V running VMs from your file share.

https://blogs.technet.microsoft.com/yungchou/2012/09/10/windows-server-2012-hyper-v-over-smb-explained/