Missing root folder issue - Taken from the Sonarr wiki as it applies here as well.

Health-Checks.md

@@ -143,3 +143,17 @@ This mechanism is triggered if Radarr was unable to get a response from the inde
 You can prevent the warning by disabling the affected indexer.
 
 Run the Test on the indexer to force Radarr to recheck the indexer, please note that the Health Check warning will not always disappear immediately.
+
+### Series Folders ###
+
+#### Missing root folder ####
+
+This message may appear if a previous root folder is no longer used. To remove old root folders:
+
+    Begin adding a new Series by searching for a name, but do not submit
+    In the 'Path' dropdown, select 'Add a different path'
+    Click the cross at the end of a row to remove the unused path
+
+Note - if the folder has been removed from the operating system, you will need to recreate the folder in the operating system, and restart Sonarr, or else you will not be able to select it from the dropdown box. After that you can remove the path from the operating system.
+
+The message will continue to appear until all series have their root folder updated to the new root folder. To change the root folder for other series, visit the series editor.