Grob

My experience (sorry if this seems extra windy... :-D )

1) If you have an externally powered device connected via 4-pin 1394 to your PC, then it is actually ALWAYS powered until you unplug the device from the wall. When a device recieves a request from the OS that requires media access (i.e. ReadCapacity, Read, Write, etc.), then in the case of rotational media such as HD, Zip, CD, etc. the device will simply spin up and ready itself for access. The device does not actually power up, but just spins up.
It may then spin back-down after some time (time usually user configurable but usually in the range of minutes), waiting for the next request.

2) If the device is indeed host powered and the host can supply the current requested by the device during enumeration, then when the device is plugged in, the PHY and LINK layers will recieve power from the host, the device will power up, interrupt the host to indicate that a new device has been plugged in and the host will then provide access to the device.

3) For 4-pin connectors, the PHY and LINK layers of 1394 have no power unless supplied by some external connection (wall plug or something). The PHY layer contains the actual physical hardware used to communicate data over the 1394 bus. If this has not power, OSes cannot enumerate the device and the OS will see no device connected.

So if I understand your question:

A) 1394 is designed to be hot-plugable. If you you've powered you laptop, powered your external device and THEN plug your device into the laptop (assuming appropriate 1394 drivers installed), then the device should enumerate properly.
The 'E' is already powered when you plug it into the PC (this is what I see in your question) and does not (nor can be if using 4-pin connector) need some other powering mechanism from the OS beyond what may have been negotiated during enumeration.
One of the rolls of power management is to simply determine, if a host-powered device is plugged in, can the host (the PC) supply the current requirelments specified by the device (negotiated during enumeration). If the host can, it will provide a drive letter (in the case of Windows). If the host cannot supply the power requirement, you will not have access to the device unless you exteranlly power it.

Does this make clearer or just muddier?
Please, reply if you need more info. :-)

Thanks

-Mike