[Error message] An unknown issue occurred and Google Drive needs to quit. [Suggested steps] Restart Google Drive.If you get this message repeatedly you may also see the message "If you see this error repeatedly, try disconnecting your account." Here's how to disconnect your account: Disconnect your account. Start Google Drive again by opening it from your Applications folder. Re-sync your files. [Likely reason] 
Google Drive for your Mac/PC quit because of an unspecified error.