I attempted an upgrade from 7.0-rc1 to 7.0-rc2 and encountered an error: "EntityFieldQueryException: Field storage engine not found." I had seen this before. But now it was stopping me from doing an upgrade, so I did some research and found that in the include/entity.inc on lines 1037,1039, and 1048 the word "sql" is missing from the field name. I changed those three lines to include sql in the field name, ie storage became sql_storage. Once that was done my problem went away. If you examine the field api and the field module you'll find the field name specified as field_sql_storage. This probably isn't the total answer to the storage engine not found error. But it fixed it for me so far.