[trap49]

Marshmallow installed fine on my N5, seems to work fine. I tried plugging into my PC with the USB cable like I always do, but the file system does not show on my computer. What is wrong?

Followup: I know Jerry wrote an article on this, but I couldn't follow his directions to "go into your notifications and tap the USB connections entry." Couldn't find this anywhere. But in the comments to Jerry's article, FisherOfMen58 posted a way to get to the options by going into Developer Mode.
In Settings, About Phone, scroll to the bottom and double-tap Build Number several times to get to Developer Mode (it took me 4 double taps).
Developer Options will now display under Settings, System
In Developer Options, scroll down to Networking, Select USB Configuration

Once it was set to MTP, I was able to see the file system on my PC.

Thanks to all who contributed, and thanks again to FisherOfMen58.

[Linas L]

The same.. :-/
I found the information here digitaltrends.com/mobile/how-to-install-the-android-m-developer-preview-on-your-nexus-device/6/