Permission errors and deleting files

Latest firmware, latest Mac OS. 

I have a large capacity HD attached to the USB port on the router, all set up correctly in Readyshare with no password set to the share (but a pw set to access the router). 

It has always been flaky with permissions. When copying movie files to the HD from the MacBook Pro, it frequently throws up permissions errors, which can only be fixed by rebooting the router. It also refuses to let me manually delete some files, with a permissions error.

When i connect the USB drive to my Mac, it is accessible but Mac OS throws up an error saying the drive is damaged and cannot be repaired.

And now the worst thing of all - yesterday, about 300 movie files just disappeared from the drive. They are literally gone. I changed nothing but every title past "R" in the alphabet has been erased.

WHAT IS GOING ON. This is appalling if I can't trust the router not to randomly delete files?! Appalling.