Personally I prefer an externally powered drive. Especially when a laptop runs on battery it gives just enough power via its USB for the external drive. So I consider externally powered to be more safe.
Generally, only a 2.5" laptop disk can do without external power. For the 3.5" disk you'll need that adapter.
And your questions:
1. No systematic difference.
2. No. Don't do it. It really needs that power.
3. Like #1.
4. Like #1.
Kees
I need an external hard drive for taking regular backups of all of my data.
When i searched the internet I got two options:
1. Porable HDD [high cost, slower speed but don't need external power]
2. Desktop HDD [low cost, higher speed but need external power]
I want to know that...
1. Which hard disk's live longer? Porable or Desktop.
2. I heared that pulling off USB cable of Portable HDD may cause data lose because it also supplies power to hdd.
What will happpen if Power goes off from Desktop HDD? Will my data/Hard disk remain protected?
3. Level of safety of data on moving or shaking on both Desktop and Portable HDD.
4. Can we install os and boot from portable and desktop external hdd?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic