AX1800 freezes after 45 minutes transferring files to USB drive.

Hi, I have recently installed an  AX1800   ( RAX20-100cns)  and attached a  WD 12 TB Easystore drive to the USB port. It connects and transfers trouble free but slows down after and hour or so until it crashes the router. For example, I was transfering a 3 tb directory and started moving the files ( large image files about 20 mb ) less than a second per file. After an hour it was moving them at 10 sec per file and then within the next hour it would be 30 sec per file and ultimately would crash. When this happens, it also kills my internet as the router seems to completely freeze. 

I was using Syncback to back up my files, and was able to get all the files moved by re-running SyncBack multiple times, but of course each time the router crashed at about 20% of the 3 tb.  

To me it seems like a firmware flaw. ?? 

Any thoughts on this?  Anyone else experience this?