Diagnose "Storages" tests if the remote Storage Devices are correctly mounted an re-connects them, if the Storage Server was out of Service.
Also checks the access rights to files and directories by writing temporary files that are deleted after the test.
The Threshold for warnings and errors can be set in the diagnose, to get information if the Storage Space is getting lesser.
Error "Used space limit of xx % exceeded!"
Free some diskspace or increase the threshold as a short solution (you need to click "enable selftest" again to use the new settings).
Error "Host down or network connection lost"
Make sure, that the storage is online again and the network connection is available.
Unmount the storage in storage manager (Admin Webinterface -> Configuration -> Storages) and remount it.
Open the containers again, that are in the storage.
If this does not solve the problem, reboot the appliance.