Cannot display data in large folders

I am using my ReadyNas 104 as remote backup (normal copy of data) in a remote location, using LAN over power-line via Devolo. Copy data run every night without any errors. I am a photographer and my photo folders are huge. The problem I am facing is, I cannot open the larger folders. E.g. a folder of 137 GB works fine, after 42 Sec I receive the data, two other folders 230 GB and 750 GB is not returning any data. E.g above the progress bar the file folder structure says..\..\data\picture\D5100\2013 and the progress bar is running, after 60 Sec. The folder structure jumps back to \picture\D5100 and later to \picture. Exactly the same happens with the \picture\2014. I am therefor jumping to the conclusion it is a timeout problem, if so can that be changed?
I have tried to connect the NAS 104 direct via LAN, however, that did not fix the problem. Please help, I am very concerned that do not have a backup of my photos for 2013 and 2014.