If you rename the Dash in a file manager (explorer on PC, finder on Mac, files on ChromeBook) from say F: "DASH" to f: "DASH-L50" or "DASHY MCDASHFACE", it'll get stuck "Exiting USB mode, please wait".
This message will persist through a restart. The only button that works when stuck is the power button.
You can however sync the device via Stages Link/Sync no problem and change settings.
Rename it back to "DASH" and all is well.
I renamed it as I was plugging in both the L10 and L50 and wanted to differentiate them.
2 weeks to work out what was happening...