Basic care maintence of my external hard drives



Is it safe to have an external hard drive plugged in & on while on the internet? I sometimes go to Google to check my spelling when typing folder names of content on my hard drives. Also what is the best way to store them? upright vertically or flat? I have 2 stacked on top of each other, is this safe? I have 3 iomega external hard drives, 1) 160GB 2) 250 & 3) another 250. I notice on my 1st 2 drives the way to turn them on is to plug them right into my front USB ports on my computer, & to turn them off, I have to unplug them from the USB port, & push a little button on the side to turn them off, I see the little blue light go off. They are in plastic enclosures. My 3rd newer one is in metal & plastic enclosure & it has a bigger switch like a light switch, that turns it on & off with out having to plug & unplug it from the USB port.

Are there any general care regimen I need to use to maintain my drives? what is the longest period of time that is safe to have them turned on? I am stuck/my PC is stationed in a hot kitchen, & I sometimes work for a couple of hours on files. How long do external hard drives last? When & if I get a new one, what is the best type? & brand? are metal enclosures safer/better than plastic? what about switch type? that turns the drive on & off?

I bought my drives in 2004 & 2005, how long can I expect them to last?