Typical power consumption of 2.5″ laptop external hard drive connected to USB socket

The problem with external enclosures is heat. Standard internal drives expect some ventilation, while external enclosures often seal the drive in with no forced ventilation.

If you want an external drive, it would be better to get something like the WD Passport, because it is already engineered with the intention for it to be used externally. Also, these types of external drives, being targeted for external use, will probably be engineered to withstand more bumping around than a standard internal drive(stuffed in an enclosure).

Now as far as your question about whether the external drive, or your laptop's hard drive is more reliable, I don't think can really be accurately answered. The best thing to do is assume either will fail at any time, and take precautions to protect your data! The problem with hard drives and reliability is it is very difficult to quantify or measure, and manufacturers generally have no incentive to invest in significant improvements to reliability. They can offer better capacity, RPM, and latency, in an effort to increase sales. However, increasing reliability through design and QA will cost them extra money and increase drive costs, and it is not something quantifiable that they can use to market and increase sales. So from one drive to the next, it's really hard to know what to expect.

Generally, no

Specifically, some enclosures can have limits relating to the SATA/IDE controller built into the enclosure.

All 2.5 inch HDDs I've seen have roughly the same power consumption (depending on 5200 or 7200 RPM mainly). Every single one I could operate with the 2 USB cables included with the external hard drive cases. If I plug it into the back of the PC I often only needed one USB cable. Using plugs on USB hubs will often cause problems, though.

USB sockets, per specification, can output up to 500 mA.

Practically 2.5" hdds could use from 600 to 1100 mA for running up. The high power consumption for this short time is - most of the time - allowed by motherboards. When running they will need about 250 to 400 mA, so they are way into the USB specification there.

3,5" hdds will NEVER work from USB alone, since they need +12V power and for a conversion there just aren't enough ampers!