How much power does a Western Digital Passport Essential SE require?

It does specify the interface as USB 2.0 which is 500mA max, the hub that is in the keyboard evidently can only support 100mA, which is common for keyboard hubs, and is most likely a USB 1.1 hub also. USB hard drives that do not have an external power adapter (common for 2.5" sized external hard drives) require more than 100mA due to the fact the hard drive has spinning platters and a motor to drive them.

I have a Dell keyboard with 2 usb ports, I connected my flash drive to one of them and got this message which verifies it is indeed a USB 1.1 hub.

.

Then I went into device manager and found the keyboard hub then I went to the power tab for that hub, which shows the max mA per port.

.

Now here is where I am baffled, I connected my 2.5" usb hard drive to my keyboard and it did not work, no message in windows, nothing, then I connected it to the USB port on the PC and checked in device manager to see what it required for power, it said 100mA, maybe it is some other limitation of the keyboard USB port or device manager is wrong?

.

Next I used a USB Y cable to get power from 2 usb ports, connected the Y cable to both keyboard ports, then connected my 2.5" drive, still nothing, the drive was cycling power spinning the drive up and down, then I took one of the Y cable connectors and connected it to a usb port on the PC, plugged in the hard drive and now it works as expected, so it must be some sort of power issue with the keyboard hub.

.

Well, there is a power specification provided by all USB devices. But if you know the USB protocol well, you will find that the power requirement provided is not calculated by the computer (host). The power requirement must be transmitted by the device to the host. So if a device sends that it needs 100mA, it may still actually need 300mA.

Since the data can be unreliable in many cases, it is better to see what the manufacturer ratings say for the device. Otherwise simply assume 500mA for relatively heavy electronics like HHD or vacuum cleaners, whatever.