I know that the eject command can be used to eject almost any hardware component attached, but can it be used to eject USB drives?
Is it possible to eject USB drives and external HDD's with the eject command?
No. Nor do they need to be;
Unmounting is sufficient for USB/eSATA/etc. storage devices.
"Ejecting" has no meaning for hardware without a tray or other loading mechanism (I assume it works with tape drives too).
However, testing with an external USB flash drive tells that