(Un)mount internal HDD at will
I'm assuming that you're talking about ejecting the drive using the the Finder. If so, then simply unmounting the disk, as opposed to ejecting it, should allow it to remain in the list of attached disks so that you can re-mount it later. For example, using

diskutil unmountDisk &lt;device or name&gt;

in the Terminal can be used to unmount the disk, but still leave it in the available drives list which can be seen by using

diskutil list

which can also be used to discover the &lt;device or name&gt; you require. Then to re-mount the disk you can use

diskutil mountDisk &lt;device or name&gt;

Note that all the above can also be done using the built-in Disk Utility application, just use the Mount/Unmount button as opposed to Eject.