You could lookup the technical specifications for you specific drive.
External drives that are not host-powered (which have to be wired to a power outlet with an adapter of their own) have at least 4 different power measurements.
- Active -- when disk activity is going on
- Idle -- when disk activity is not going on, but the drive is attached and powered
The drive may or may-not be spinning at this time (that could be two different sub-states).
- Standby -- when drive is Idle, Attached but has gone into a standby power & wear saving state.
This is when it takes a little longer to access it again.
- Host-shutdown -- this is when the host-PC is shutdown but the drive continues to be powered through its adapter.
This is similar to any other appliance in its 'standby' mode where it is shutdown but the power outlet is still ON.
The tech-specs for these drives will usually show reducing power requirements in ascending order for the above points. And, if the drive does not perform on one of the latter points, it would typically be excluded from the tech-specs (think of that as the fine-print).
As a bottom line, the states of your drive where the disk-platter is not spinning (assuming you are not talking about solid state drives here) will have relatively lower power consumption compared to when it is spinning.
This difference can be usually perceived by keeping your palm on the top of your drive (if its close by). A spin will always show up as a mild vibration.
The Maxtor (Seagate) drive you refer should have a good idle/standby control.
And the power consumption would typically fall significantly once it spins-down.
I hazard it be be around 2w tops after spin-down, but don't go by that :-)
So, it should be fine to leave it wired and powered while your PC is on.
Unless, you keep your PC on most of the time and use the drive very rarely (or do not turn off the drive power when you shutdown your PC) -- in that case, extra efforts towards powering-down the drive when not in use would be worthwhile.