Virtually disconnect an internal hard drive.

Well, first, since the HDD is SATA, then per the ANSI specification, you can remove power & data cables in any order without hurting the drive.  So from an electrical perspective you won't damage anything.  

But if you want to do this without hurting data, you need to dismount the HDD (remove the drive letter), then wait 30 seconds to insure all data has been flushed from the O/S to the HDD and then anything in the HDD cache to disk.    Then after the data has been flushed, remove either the power or data cable or disconnect one end of either.   (Two won't hurt, but you have to remove at least one of the signals.

Hi Steve,
You do this by dismounting the drive (and to gain back access to it, you mount the drive). Here are detailed instructions on how to mount and dismount drives:
http://windows.microsoft.com/en-us/windows7/mount-or-dismount-a-drive

Regards, Joe

Internal hard drive are not swappable.  I would recommend a USB docking station or hot swappable drive bay from here: http://www.thermaltakeusa.com/storage.aspx

Another approach would be to install a switch for the power to the internal drive,
http://www.amazon.com/ORICO-HD-PW4101-Floppy-Switch-Control/dp/B009XPCL5U/ref=sr_1_7?ie=UTF8&qid=1373177621&sr=8-7&keywords=hard+drive+internal+switch

Steve,
A slight change to my earlier comment – you do not want to mount it in an empty NTFS folder to get it back...instead, select the radio button to assign a drive letter. It looks like this:
Regards, Joe

If you are just unplugging your external USB disk's cable when you don't need it, then you are risking it's data, as this action can cause file-system and data corruption. If you are using windows as OS, you must use the "safely remove" utility and wait for the confirmation that the drive can be removed. With other OS's you have similar options.

For internal disks, you could just disable the disk in the PC's BIOS for those times you don't expect to use it.

Start > Run > 
 Type      Diskmgmt.msc

When the list of disk drives shows up,  left click    the volume in the bottom pane.    to the left of     blue, orange, or purple bar.

For some kinds of disk drives,  you can right click the drive and there will be an "Offline"  option.      If you select this,  the disk drive is taken offline.

Otherwise  change disk drive letter / mount point assignments,  and remove the drive letter from the disk drive.


If you go the route of unmounting or  removing letter assignments;       in  some sense,  the disk drive might  be at greater risk    for accidental error ----      the disk drive is no longer mounted,   so  some maintenance tasks    may produce fewer   "Are you sure"   warnings,  than  they would for a busy mounted disk drive.



Of course, if you are willing to  open the machine,  you could always open it up and unplug the disk drive's  data cable and power cable.


If you are willing to power off the machine as well,  or incur downtime,  you  could  just power off instead of unmounting.      There are some removable dock  options which would allow you to physically remove the disk drive from the machine as well,    without opening the chassis.

Others pointed out AHCI provides for hot swapping sata drives, but not the drive that has the OS or a page file.

Yes Joe, that's exactly what I'm looking for.

Steve,
I'm glad to hear that's what you're looking for. Regards, Joe