I have an Iomega external hard drive that has a power ON/OFF switch on it...So my question is, can I leave it's power adapter plugged and turned on (while HDD not using), when I turned the HDD power switch turned OFF?
Is it safe?...If I do that, power adapter will get hot?
Because I do have two different ext. HDDs and I use them when I want but it's really annoying unplugging when not using.

All the wall wart type power adapters are always "on" regardless when they are plugged in. That is one way they use electricity all day long even if the Iomega drive is not powered on. The power pack is on. Yes you are at risk if it's plugged in. Any power surge can take out the AC Adapter at anytime. Best thing is to plug them into a UPS or a power strip plugged into a UPS then turn off the power strip. Having all your computers ext hard drives on a UPS is the best protection you can hope for.