I downloaded a selected sub-set of the USGS National Elevation Data (NED) using their online GUI. This download came in the form of three files.
The names of the files are 61471006, 61471006_2, and 61471006_3.
To load the rasters, I have found I go into these files and load the file of type ".adf". (Is this not true?)
It works just fine for one file at a time. When I load all three in turn, the QGIS layers list shows three little raster icons with tiny "previews" of the area covered; the name of each one happens to be identical to the other two (w001001).
They always load "flat" -- that is, one gray-scale tone. So I use the Properties dialog to stretch to MinMax. When I do, only one of the three will change and show the range of shadings indicative of elevation range. The other two turn black.
How can I get all three to show the MinMax range?