The way that worked for me on the PI (3 "B") was to install raspbian which is
the default in the SD card installer that came with the unit. From there follow the
instructions (see other thread) to install. Should be no need to build gnuradio from
sources unless there's some custom reason for doing so. Certainly not necessary for
OP25. No way the process should take three hours, but the limiting factor is the
connection speed since it has to download a bunch of packages...
Max