Is the folder path still correct e.g. does the file show as being valid when you try to open the file for information via alt+3 or from looking at the playlist entry via the ctrl+e dialog on a selected playlist item?
As my initial thought is that the path is somehow different or there's maybe a permission issue from the moving of things that hasn't persisted which is preventing the access or they're in a file format that the wacup install is no longer configured to support (as it's not clear if you also made a new wacup install or migrated it as part of the move to the new hdd).
OK let me get it a little more clear.
The HDD that I changed was not the one with the OS, so I didn't install or reinstall anything.
I changed one disk that was 2TB with a new one that is 4TB.
The name of the new disk is the same as the old one.
And I just copied a big sized folder from one disk to another. Most of the files are ok, except this particular folder and its sub-folders.
Both alt+3 and ctrl+3 work fine.