While exploring the new Business Connectivity Services from SharePoint 2010, I created an external content type with only 2 methods specified (ReadList & ReadItem). As the only 2 methods available are Read methods, the external list only displays data from an external content source.
After I added a Create method and updated my wsp, I received this error: in my eventlog:Unable to find the default new form for list
The list was created by using the readonly contenttype and therefor the list did not have a NewForm associated. After deleting this list and recreating the External list with my cutom external contenttype, everything went as smooth as it should!
So next time you create a new Create or Update method on your external contenttype, don't forget to recreate the list in your sharepoint 2010 site.
Oh, you don't have to worry about any data loss 😉