ian,
For ECW file (created by ER Mapper or ERDAS Imagine), users very often see this type of black or white edge issue. Right now, Hexagon Spatial (ERDAS Intergraph) usually recommend users to use JP2 (rather than ECW) for interoperation.
In order to remove the black or white edge issue around the ECW file, you may try below:
1. to import ECW into IMG by ERDAS Imagine, then clip dirty edge of this IMG file in ERDAS, or manage this IMG dataset in MD via manually editing footprint (preferable);
2. directly add ECW into MD and then manually edit footprint in MD to remove white edge (not recommendable)