Will ejected internal HDD really stay ejected until I mount it again?
I suggest to test and use:
    Disk Arbitrator.

This is a small application interfacing the MacOS X disk access interface.
This application may be used in forensic investigation, to protect against unwanted connection of external disks, and to manage very straightforwardly any mounting and eject of internal or external disk
(through the Show Disks Window menu entry).

I wouldn't speak in place of the author, but I can ascertain this tool is working on: Lion, Mountain Lion, Mavericks &amp; Yosemite.

I haven't found any bug in this application, and I would be glad to give the same nul result with many application I have to survive with :(.