Disappearing attached storage on D7800

[MACH100]

I have a D7800 Nighthawk router to which I have attached a 4Tb USB drive to provide a basic NAS resource for sharing files on our home network.

Various folders on the drive are mapped as drive letters for easy access, and all was working fine until a few months ago, when I found I was no longer able to access the folders.

Now, if I try to access any of the mapped drives, I get an error message similar to this:

I have checked the settings in the configuration of the router, and they still seem to be present. The folders are accessible without issue if the USB drive is removed from the router and attached directly to the laptop, so the drive is not at fault.   I am wondering if either some Windows 10 update or possibly a router update may be the source of the problems.

Can anyone suggest what the problem may be ,and hopefully how it may be solved?   The basic NAS that I had was absolutely fine for my purposes, and unlike an older 'true'  Buffalo NAS unit I have, it does not involve powering up a HDD unite 24/7, which is not appropriate for the frequency of access required.