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Tonido software requires external storage to store application data from apps like Sync etc. Without a connected HDD, Tonido software will not function properly.
Tonido has a setting called "User Data Directory" under Settings->Account and it has to be setup to point to a connected external storage location. This location will store all Tonido application data that is not stored on the TonidoPlug internal storage. If for any reason, you disconnect a HDD connected to the plug or if the disk is not connected properly, this location is unavailable and all applications will be SUSPENDED.
So, ensure that a HDD is connected and is available for browsing under the "USB Drives" section.
Next, go to Settings->User Account and make sure that the location set under User Data Directory is OK. The default location is /media/disk1part1. If your HDD is mounted in a different location, make sure to change the path to the new location. You can re-enable the applications after setting this correctly.