I'm trying to use a number of raster functions in QGIS version 2.12.1 Lyon under OS X10.10.5 Yosemite and repeatedly get the message that "The process failed to start. Either the invoked program is missing, or you may have insufficient permissions to invoke the program." Though I can use Georeferencer and many other functions happily, neither Merge, nor Clip, nor Build Virtual Raster work. I assume that this is something to do with the long-running problem of links on the Mac to Python, but none of the previous advice I've found seems to be relevant to my case. How do I get round the problem?