Can't open folder on external drive, but can see it

Hello,

I copied a folder to my external drive a few weeks ago and today when i tried do access it i can see the folder on finder, but i can't open it or even see it's contents on the right side of finder.

The strange thing is that i'm duplicating the folder and in there i can see the contents. Haven't tried to open any file but i'm presuming that i can, because now i can see them on finder.

Anyone nows why this is happening? The external disk is a Verbatim, 320 Gb and it's formated in Mac Os Extended (Journaled).

I want to know so i can prevent this from happening and because i'm thinking of using it as a backup.

Thanks!