Yes, on both USB 2.0 and 3.0 ports. The is applies to files sized in 10's of kilobytes so speed of transfer do not seem to be a factor. I tried with a formated usb 2.0 stick and had a same problem.

Thanks but it is not the case unfortunately, when HDD is connected to PC file access and modification is normal. Tried connecting another blank usb drive to router, created a new word document, tried...

Thanks in advance for any help. I Have windows 10 X64 build 1903, fresh clean install with stable drivers, connected to router through 1Gbps LAN cable. Connected ExFAT 1TB HDD to USB ports on router...