@billesan 

I would say this is likely permissions as its one single folder.  Possibly your PC has the correct credentials to acess this folder others dont.

Try renaming the folder then create a new one with the old name, if that accessable then copy your data accross

Did you check the permissions?

@Philbert Thanks for the creative suggestion. I've fiddled around with it some more, based on your suggestion.

What I did was:

1. Connect the drive to the router. Create another folder (as a backup) on the network drive

2. Connect the drive directly to a USB port on my pc. Copy all the files in the troublesome folder to the new folder

3. Connect the drive to the router. Try to access the new folder (this still did not work)

4. While the drive is still connected to the router, access the drive via FTP and navigate to the new folder. Delete files one at a time, each time trying to access the folder via Windows Explorer

Based on this approach I found that the problem only occurs when there are files above a certain size in the folder.

I'm not sure exactly what the threshold is, but image files with a size of >3GB causes problems, while image files with a size of >1GB are fine. Permissions on both the folder and the files are the same throughout, even the same as folders with no issues.

So it seems, I guess, that there is a limitation somewhere preventing me from accessing folders with files exceeding 2GB in size.

The drive is already formatted as NTFS

@billesan 

Sounds like the file system restricted to 4gb errror you get with FAT, but should not be the case with NTFS

If you have a spare pen drive around try formatting it as  exFAT  and copy the 4GB files to it then retry..   cant quite think why that would cause it but worth a try!!

If you feel curious of course!

@Philbert Thanks, I may give that a try. I will update this thread if I find a solution