I have a MacBook Pro (15-inch, mid-2012) with two hard drives in it. I took out the DVD drive and replaced it with a second hard drive. The primary hard drive contains all my applications and data; the second hard drive contains an operating system and a set of tools I can use if my first hard drive takes a header, as well as backups of critical data I have not yet backed up to an external drive. (I'm paranoid and meticulous about backups.)

I'm replacing the primary hard drive with a 1TB SSD--mostly for speed, but also to extend battery life, as I often travel long distances with this computer. An SSD, lacking a motor to spin the hard drive platters, uses far less power.

I want the second hard drive, which is still an old-school spinning-rust jobbie, to mount only when I need it. To conserve battery power, I would like it not to auto-mount when I boot from the primary...but,, and this is the important bit, I still want to be able to select it as the startup volume if I option-boot the computer. I want it to not auto-mount so I don't have to remember to eject it to save battery power.

Is there a way to do this? I suppose I could write an AppleScript to dismount the second drive when I boot from the first, but that seems clunky. I'd prefer a simple, clean, elegant way for it not to mount at all but still be available if I start the computer with the Option key down. Any ideas?

I thought I had bookmarked something that did what you're looking for, but no such luck, although it sounds familiar.

Searching MacUpdate turned up MountWatcher ($18) which may be useful if I'm understanding it.

It seems to allow you to set a separate user name and password for each volume, so (presumably) dismissing the p/w pane with no entry would prevent your drive from mounting.

Ah ha! Found it... Prevent a volume from mounting at boot, but I don't know if it affects an option-boot. (Also, it's from 2012.)

Hope this helps.

Thank you! That's exactly what I needed. It works a treat.

(You've obviously spent some time in the UK lately. )

should be trivial (and free) to just create a login action that unmounts the drive? I'd make a launch agent, but others could just make an automator action I suppose and drop it into their Login Items

What he thanked me for was the link to a Terminal command, not the one to the $18 app (which I would have edited out had he not posted first, because it's even less elegant than the AppleScript he proposed).