02-23-2011, 11:48 PM
they provide extra storage space.
you wouldnt store your OS on the external hard drive. you need to keep that on the internal drive, because the data transfer rate is much higher. if you put your OS on a USB or firewire drive your computer would be alot slower. (plus, you would have to buy the SL retail disc anyway, but we wont go there
you could set up your external as a time machine backup: this saves all your data, OS,documents etc, everything! so if your hard drive fails, all your data is safely backed up.
you could however set it up as just extra storage for when you need it, just dragging music or movies over etc.
external drives are fairly cheap now, 250GB ones being under £50 here, so it should be about the same in the USA. why buy an 80gb one when you can get a dirt cheap
I am a Seagate fan, never had problems, some people on here prefer Western Digital. its really just personal preference/experience.
hope this helps somewhat.