I have a MBA and have finally updated from Yosemite straight to Sierra (missed El Capitan). Now, I have files stored locally and I also have a HDD plugged into an Airport Extreme which, thru my internet router is connected to my Air. All the files stored locally on the MBA are fine. BUT - when looking at the external HDD via the AE I can see my files, and I can even preview them with the spacebar. BUT I can't open them by clicking on them. Instead I get a message that reads "The application "XXXX.app" can't be opened" and then there is -36 in a smaller font below that message. XXXX refers to any app such as Preview, Microsoft Word or Microsoft Excel. Here's the weirder part - I CAN open all of these files if I open the relevant app first and THEN go to File---Open and select it from there. And even weirder part? There are a few pdf files that open in Preview the normal way, yet others that get the same error message, along with the jpgs, etc. My wife's MBA is still running Yosemite, and it is fine. And if I plug the HDD directly into my MBA it is all fine so it is something to do with the AE. I have checked and there are no software updates. I have also checked that the correct apps are assigned to the files and they are, which is proved by the fact they open when the AE is not involved. I have already tried to delete the relevant plists but this hasn't made any difference. Unless there are plists I haven't thought of. I am at a total loss. Help please!