We can probably guess that the enclosure contains one of WD's Scorpio drives. According to this page on the WD website, the 1TB Blue Scorpio consumes 500ma while reading. So the total draw of the drive + enclosure shouldn't be very much more than that.
I would guess that the drive is designed to run within the power envelope permitted by USB 2.0 so that it doesn't require a separate power connection.
1) max current capability for USB2 is 500 mA. If the drive is rated @ 500 mA for read, A) yes the surge current will be higher and B) the write current will be higher. USB3 increases the current from 0.5 A to 2.0 Amps.
With portable 2.5" drives I always use either (A) a two plug cable (plugs into two USB2 ports, and that may require a enclosure that supports that, don't know for sure OR (B) a USB hub that allows for a ac adaptor to provide the power. Yes I've also had problems with the portable USB 2.5" drives drawing too much power from a single usb 2 port. Luckily my newest laptop has two USB3 ports and my WD USB3 drive runs fine plugged into it, but when plugged into USB2 I use a powered hub.
Yes chief (greenie perhaps?) I guess I could use a hub but what attracts me most
to these drives is their simplicity and small size. Fortunately I don't HAVE to use them like this so I can avoid the extra hassle of hubs etc.
What I have noticed on the PCI Express card is a very small power socket that
isn't even mentioned on the scrap of paper that came as a "manual" with it.
Maybe I'll break one open and see if the circuit looks like it accepts power. Maybe
thats how they made a dual port device work in the first place.
Still - not an end of the world issue. I'm just curious.
Most USB 3.0 adapters for the PCIe bus have a separate Molex or SATA-style power connector on them that you plug one of the power supply leads into. This is because USB 3.0 permits 900ma of current per device, and for a 2-port adapter that's more than it's allowed to draw from the PCIe bus slot.