I've got a ImageMosaic (GeoServer) created from 4 band *.sid files. Images become transparent around the outer edges of *.sid files . When 4-th band is removed with
gdal_translate -b 1 -b 2 -b 3 -of /X/Y.sid /X/Y_2.sid (
gdal of course says
Warning: The target file has a 'sid' extension, which is normally used by the MrSID driver, but the requested output driver is GTiff. Is it really what you want? link to a question about this)
geotiff the images are displayed as expected but overhead storage required for geotiff files is unwanted.