I was just trying to backup my whole wordpress 3.0.5 database via shell when I got this error when all plugins deactivated:
mysqldump: Got error: 1016: Can't open file: './my_database/wp_7_rg_lead_detail. frm' (errno: 24) when using LOCK TABLES
Then I tried again with all plugins activated:
mysqldump: Got error: 29: File './my_database/wp_7_rg_lead_detail.MYD' not found (Errcode: 24) when using LOCK TABLES
So it seems to be something to do with gravity forms? Never happened before.