On the subject of RPi and distros/OSs, what media do people use to run their
OS for hte RPi? Is it cards? has anyone attached regular drives? And what is
the favourite way of straight-up tweaking the RPi's OS before deploying it?
And, lst one, has anyone powered the RPi and a harddrive from a common power
supply/battery?

I am interested in having a robust filesystem, as in, having proper shutdown
procedure when power on the RPi's primary source is running low (say due to
a power outage of the UPS). I would like to power the RPi and a regular SSD
or HDD from a single source.