Browsing files on my network sometimes lags and sometimes give me errors?

I'm using my MacBook Pro on my network to go through files on my iMac, specifically, files on an external drive connected via Thunderbolt to the iMac. It usually works fine, though sometimes there is major lag, and when I open folders nothing shows up. I even have deleted some folders thinking there was nothing in them, though now I realize I may have deleted files I wanted not knowing they were there!

Now, when connecting it is showing a red dash in the lower right corner of each folder and when I try to open the folder it indicates I don't have permission.

What is going on?

Thanks.