Issue: After modifying the search topology you may be seeing one of the search component with degraded icon as shown below.
Reason in this case is that I have changed the search service account, when you provision the search service application with the PowerShell script – central admin app pool account will be used as Search service which is not a good practice.
after changing the search service account – index location changed the icon changed to degraded.
Microsoft.Ceres.SearchCore.Journal.JournalComponent[Configuring] Component failure: Failed to configure component Microsoft.Ceres.CoreServices.Services.Configuration.ComponentConfigurationException: Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED)) —> System.UnauthorizedAccessException: Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED)) at System.Runtime.InteropServices.Marshal.ThrowExceptionForHRInternal(Int32 errorCode, IntPtr errorInfo) at Microsoft.Ceres.CoreServices.Storage.Util.StreamFactory.CreateFileHandle(String path, FileMode mode, FileAccess acc, FileShare share, Int32 fileOptions, Int32 blockSize) at Microsoft.Ceres.CoreServices.Storage.AbstractD
Looking into SharePoint 2013 logs – after modifying the search service account, WSS_WPG group doesn’t have access to the Index location. (In my case I have changed the index location from default to a different directory).
after assigning the WSS_WPG group Full access to the index location , index component status back to normal !!.
Hope this helps some one :-).