Update Troubleshooting.md

Documentation/Troubleshooting.md

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 ## About video caching
 
-- Change cache location : This option simply changes _the location_ of the Cache folder that Aerial uses. It does _not_ move your files for you. Please note that this change will only be taken into account the next time Aerial starts (you may need to fully close System Preferences). We strongly recommend you use a path that's always accessible, as Aerial **can't work** without a Cache directory. In case the path is no longer available (missing USB key, etc), starting with Aerial 1.4.7, it will reset the Cache location to it's default location (usually `/Library/Caches/Aerial/` or, if unavailable `~/Library/Caches/Aerial`) to avoid crashing.
+- Change cache location : Starting with Catalina, and because of the sandboxing limitations introduced with macOS 10.15, Aerial will use two distinct folders. Because of the sandbox, Aerial can **only** write inside the sandbox. You can however still specify a secondary cache location, this is what the cache location is about. This is a read-only folder where you can move your videos if you wish. You need to do this manually, changing the cache location **will not** move your videos as Aerial cannot write outside the sandbox. Please note that locations outside the main disk (including networked and external drives) are not allowed. This, again, is a macOS 10.15 limitation that we can't workaround.  
 - Videos keeps disappearing, Aerial may not restart once in a while: Aerial stores all it's data in a Cache folder. This cache may get deleted by some third party software trying to free disk space. If you use such a "Cleaning" tool, we recommend you set a manual folder location in the Cache tab of Aerial. For example, you can create an Aerial folder in your User folder, and point to it. This will ensure Aerial files don't get deleted.
 - Black screen: If you are behind a firewall (Like Little Snitch or Hands Off!) try creating exceptions for Aerial to allow it access to Apple's servers. Be sure the applications `ScreenSaverEngine.app` and `System Preferences.app` are not being blocked access to `*.phobos.apple.com`, `*.phobos.apple.com.edgesuite.net` and `sylvan.apple.com`. If that isn't an option, please look at the [Offline mode](OfflineMode.md) documentation.