Well actually i think I'll disagree, my advice is in most cases the internal drive should do the job.Why:
a)Internal drives are designed to last longer,believe it or not an internal drive it gonna last much more than an external drive after everyday use.External HDs have even become increasingly unreliable the past years...
b)External drives are more expensive so even if they lasted the same as internal ones (which they don't) it's much better if the cheap drive breaks and not the expensive one.
c)"if anything, better to wear out the drive that doesn't have your operating system on it"
Also disagree with that.When did the operating system became more important than your files?.Windows is completely replaceable are your files replaceable?I 100% prefer to lose windows from losing 5 years of quality movies and music carefully organized...
d)Usb is a crappy connection to have always running...The good thing about Usb is that it works straight away,the(main) bad thing is it requires lots of resources so it will be constantly taking up memory and even more Cpu which will overall make your system slower.Usb is not a good connection for a Hd you wanna use all the time.Sata is and most internal drives nowadays use sata.
e)Well e) is some stuff that don't deserve their own paragraph:
i)Lot's of shit can happen to an external Hd just because it's external,dust,fluid spilling,nearby magnets(!),someone on the house doesn't see it and kicks it like a football,it falls from the desk and lots more...
ii)External Hd overheat,that reduces their stability and their speed which btw:
iii)Is a LOT slower than an e-sata internal drive.
iv)I'll think of something to add up here in the future

AND FINALLY:
f)F stands for me (hope I'm not breaking any forum rules)FUCKING hating cables,usb can't provide enough power for an hd to run properly so you need an external source of power that's 2 more cables (including the usb one) and the hard drive that take up space and are always in my way.
Btw:I'm not sure everything I'm saying is 100% true,if someone thinks I'm wrong somewhere please feel free to correct me.Also note eventually no hard drive is really reliable and
will fail at some point,so
backup is necessary until HDs find the technology to call themselves truly reliable,in your case i think your data will be more safe in an external drive that just
holds stuff instead of working all the time...
Hope i helped.