Why is it unsafe to create a bootable USB flashdrive with OS X? What about an external 2.5" hdd?

[rodion15]

I heard that creating a USB flash drive with OS X on it can damage a mac computer (and I guess a PC as well), it would damage the motherboard (the firmware I guess). I also heard that doing the same using a 2.5 inch external hard drive is also unsafe. I know that installing OS X on an external 3.5inch hard drive is safe, in fact I use one with several OS X versions for diagnostic use.
Why is that?