Readynas Duo network error terminates file copy

I have been trying to copy a 109GB file to my Readynas Duo for several days now.

It gets from 1/3 to 2/3 of the way through the copy and then I get a network error messege with retry button.

Clicking retry, just starts the copy over, asking if I want to overwrite the file on the disk.

I can still access the drive with Raidar

I have disabled disk spindown and sleep on the PC which is the source of the file copy.

Checking the log files, I found the following in smbd.log:

[2020/08/29 12:13:06.422460, 0] lib/util_sock.c:680(write_data)
[2020/08/29 12:13:06.461397, 0] lib/util_sock.c:1441(get_peer_addr_internal)
getpeername failed. Error was Transport endpoint is not connected
write_data: write failure in writing to client 0.0.0.0. Error Transport endpoint is not connected
[2020/08/29 12:13:06.462019, 0] smbd/process.c:79(srv_send_smb)
Error writing 39 bytes to client. -1. (Transport endpoint is not connected)

Of course a file this large takes hours to copy, but I have copied large files to it before (up to 3x larger) and not had this (repeated) problem. While the disks are 85% full, Raidar says there is over 5GB free space. 

Does anyone here know why this is happening and/or a fix?