I have been having a problem with a couple of different USB devices that I occasionally connect to my Mac.  I would attempt to eject them and they would immediately remount themselves.  Therefore, it was almost impossible to do a “clean” disconnect of the device.  If you’ve had this problem you’ll know exactly what I’m talking about, and if you haven’t then you don’t need this article.  If you choose to use the information here, bear in mind that even though it worked for me I have no idea what it will do on your system, so use at your own risk or don’t use it at all!

I found that I could “Unmount” rather than “Eject” the devices using Disk Utility and then they would not automatically remount, but it was a bit of a pain to have to go to Disk Utility every time I wanted to unmount a device.  Automator to the rescue! Here are the three steps to create and use an “Unmount” service in OS X:

1. Fire up the Automator application and when it comes up, tell it you want to create a Service:

2. Create an Automator workflow exactly as shown here (click on the image to enlarge it).  Note that the top part must say “Service receives selected Folders in Finder” and after that there is one step, “Run Shell Script”, in which you’ll pass input as arguments. In the text field simply put hdiutil unmount $1

Save the workflow using an appropriate filename (I suggest “Unmount”). It will be saved in your ~/Library/Services/ directory.

3. To use the service, open your /Volumes directory in Finder and select the volume you want to unmount, and right-click on it (or however you bring up the context menu in Finder on your system). Near the bottom you should see a menu selection for “Services” and in a sub-menu you should find your “Unmount” service:

Click on that and your volume should be unmounted (if you have Hardware Growler installed from any version of Growl, including the free forked Lion version, then you should get a Growl notification of the unmount). I will note that this has no error checking other than that built into the command-line hdiutil program, so while it probably won’t hurt anything if you try to unmount something that’s not unmountable (such as a file), I’d still try to be careful.

If you have a problem with devices that refuse to unmount the then you could use the same procedure to create a “Force Unmount” service. I’d still create the regular “Unmount” service as shown above, but if you sometimes have the problem that the device won’t unmount the normal way then simply follow the above steps again except name the service “Force Unmount” and add the -force flag, like this:

hdiutil unmount -force $1

Or if eject normally works, but sometimes you want to force it, you could create a “Force Elect” service using:

hdiutil eject -force $1

I do not guarantee anything with regard to the use of the -force option (I read about it here), so if you go that route and lose data, don’t blame me.  As I said, you use this stuff at your own risk.

Happy unmounting!  And if this doesn’t work for you the way it worked for me, I’ll tell you up front that I have no clue why, and that’s partly because I don’t understand why a plain old eject attempt fails on some systems (as I say, eject actually does work, but then the device immediately remounts).  It’s all a mystery to me!