1. I'm not aware of any 3.5" enclosure that doesn't need external power.
2. I use a vantec nexstar 3 enclosure with a WD 250G black drive which obviously uses more power and generates more heat than a green drive and even under load, the enclosure gets warm. The enclosure is aluminum, so it's suppoesed to get warm so that it can dissapate heat into the air, and away from the drive.
I'm with Hawkeye22, I don't know of any 3.5" drives that are efficient enough to draw power through the USB bus.
I also use VanTec Nexstar 3 enclosures, and I've found that the green drives run considerably cooler in them than 7200rpm drive. I'd describe the 7200rpm drives as "hot" and the green drives as "warm".
Can't do it. USB 2.0 provides, at most, 500 milliamps at 5 volts. The Caviar Green 2 TB drive draws
Current Requirements 12 VDC
Read/Write 296 mA
Idle 243 mA
Standby 3 mA
Sleep 3 mA 5 VDC
Read/Write 503 mA
Idle 172 mA
Standby 160 mA
Sleep 160 mA
Even the 5V draw is slightly over, and there is no way that USB 2 can provide the extra 3.5 watts for the 12 volt line, even if you stepped it up with a voltage converter. That drive draws 2.43 times the power that is the max spec for USB 2.0.
Maybe, maybe not; I'd say it depends on the USB port and the MB.
According to : http://en.wikipedia.org/wiki/USB2
Max. current 500-900 mA @ 5 V (depending on version)
However it is possible that every current demand above 500 mA must be short (transient) otherwise the port couldn't provide it.
There is one way to make sure: to test it if anyone has a caviar green and a 3.5" enclosure (just don't power it on and see if it works).
There is more chance that the 1 TB version (drawing less power) could work.
900 ma at 5 V is 4.5 W, and is spec'ed for USB 3.0 anyway. The drive can draw 6 W. Going out of spec is risky, even if it works with one particular drive in one particular case. It may work one day, not work the next, and corrupt your data the third.
I had this experience with an out-of-spec 2.5" drive.
The 1 TB model specs state: Current Requirements
Read/Write 5.40 Watts
Idle 5.40 Watts
Standby 0.40 Watts
Sleep 0.40 Watts
So my somewhat informed take is that it should not work, and if it does, it will not be reliable and should not be depended upon. Heck, I have a 2.5" external drive that requires the use of a Y cable to draw power from two USB ports. It has occasionally spun up on one port worth of power, but I would not use it that way.