Cannot install update because startup folder is not writable by the user
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Check the folder permissions. Can your user read/write the folder? The default startup folder when installed under Ubuntu 14.04 is /opt/NzbDrone/ but you can verify that by browsing to the System/Status page in Sonarr.
If the user doesn't have write access, the simple solution is to make your user the owner of the startup folder. Where user equals the account that runs Sonarr, run this command:
sudo chown -hR user /opt/NzbDrone/