Anyone knows how can i open a .rrd file (should be a raster)in QGIS 2.6? Comes with more 3 files. Should be any difference using it in OS or Windows?
Comment converted to answer and slightly extended following @AndreJ's suggestion:
The RRD (Reduced Resolution Dataset) is a proprietary pyramids file. ArcGIS will make them for any raster not just IMG. Don't try to open it as it is not compatible with GDAL (the primary underlying raster tech in QGIS) and does not contain the original data anyway. The only file you should attempt to open is the TIF file itself. The AUX contains statistics and some geopositional information. It is where Arc stores the results when you calculate statistics on a raster. The AUX is also proprietary.
Most GIS packages will generate a pyramids file either automatically and/or on command. If the raster contains the georeferencing in its own header, then the AUX can also be safely ignored because, again, the statistics information can be regenerated automatically or on command by most GIS packages (since QGIS is sort of a 'meta-GIS' containing more than three other raster technologies, it is very well provided for in that department).