Okay, just tried it - eSATA port from the spare internal port on SATA PCI card, PowerMac G5.
YES - Drive can be ejected via right mouse button and powered off, no problem. I think.
NO - Drive doesn't auto-mount unless it is powered on before system boot, so can't just turn the external drive on anytime and see the drive on the desktop. Unless there's a workaround.
NO - Doesn't work with multiple drive enclosures. One port = one drive only. I have a four drive SOHO enclosure with in-built port-multiplier so all four drives *should* work via a single USB or eSATA connection, but only the first drive mounted. All four drives mount with a single USB port, but not with this SATA->eSATA port converter so I assume it apparently needs a 'proper' eSATA port. So not the solution I was hoping for.
PS. The drive appears as a normal internal drive, not an external drive (with eject icon).
Cool!
And I'm pretty sure (99%) that we can get a drive that wasn't present during boot to scan in and mount. It guess the code to do so isn't very complex.