Problem now resolved but probably worth sharing.........
While transferring large files and folders of multiple files from Windows 10 across to the OMV folders the speed of transfers, initially beginning at around 60-80MB/s, slowly began to grind to a halt! It not only ground to a halt, I could only restart it again by resetting the Ethernet adapter in Windows. Smaller files transferred one at a time no issue (and big files up/downloading from the internet are no problem). The two machines are hard wire Ethernet connected both using gigabit adapters built in the Asus motherboards on each machine.
I began to suspect SMB/CIFS, the OMV system itself, incompatibility between Windows and OMV, you name it. I updated the Atheros Ethernet adapter drivers forwards and backwards - no difference. I checked/adjusted Atheros Advanced properties/settings for flow control, receive/transmit buffers etc - no difference. I googled until I got boggle-eyed. Big/fast transfers between drives within the Windows machine were fine, and OMV to Windows seemed OK, the problem was from Windows to OMV. I thought it might have been the Windows paging file too small to handle the big files I was transferring, so I set it from the standard 3GB to 10GB. Initially that seemed to do the trick and the transfer speeds seemed to hold at 50-80MB/s. Next day however the problem was back! And I was back to Google research!
Eventually I found one posting on a forum somewhere that pointed to it. In the Atheros adapter Advanced properties there's something called Task Offload which has a default value of ON. I hadn't messed with that as it didn't look relevant. I finally discovered that Task Offload, when ON, allows Windows to offload some comms transaction processing to the Atheros adapter. Unfortunately the Atheros L1 Gigabit adapter is not up to the job, and falls over under high loads! When that happens you have to reset the adapter (turn ethernet off then back on) to get it to work again. However - turn the Task Offload setting to the value OFF and it all works fine! Big/fast file transfers between machines are now handled without a problem. I've noticed the speed is slightly less (around 10-15%) that the initial maximum when Task Offload was ON, but that was no good when it gradually ground to a halt! So I'm very happy. Hope this helps others to avoid this problem with the Atheros, and maybe other adapters.
fop