Hard drive died... How do I reinstall Mountain Lion onto an external drive?

Sorry, I'm confused.

The recovery partition from Tech Tool, this is on the internal HD or on a CD/DVD?

Is this the same thing as the TechTool disk that your computer can boot into?

Where is the Disk Utility that cannot see your hard drive? Is it on the Apple recovery partition on your internal drive? Are you saying that Disk Utility on your Apple recovery partition cannot see your boot partition?

It sounds like your internal boot partition won't work, but the internal drive itself is OK (i.e., Disk Utility, on your Apple recovery partition, doesn't see the normal boot partition), or at least OK to the extent that the Apple Recovery partition can run.

If you install Mountain Lion on your external drive and use it as the boot drive for your computer, it will be very slow, and you won't be happy. I'd forget that idea.

Assuming I am thinking correctly about your situation, I would try to repair your HD. Your options would be:

1. a safe boot Mac OS X: What is Safe Boot, Safe Mode? and then disk utility
2. boot into single user mode and see if you can repair your hard drive Repair Your Hard Disk in Single User Mode | Everything Macintosh
3. internet recovery OS X: About OS X Recovery
4. recovery disk assisant, if you have made one already OS X Recovery Disk Assistant v1.0

If none of that works, then I'd try and wipe the entire drive and start over. I'd try to do that from internet recovery. Hopefully one of your external drives has backups on it.

If that doesn't work, then I'd replace the drive.

One question first, what happens when you boot up holding the option key, into Startup Manager, can you see/boot from your OS X volume?

If that's not so, follow arthur's good advice. Use Internet Recovery, if Disk Utility does not see the volume or reports the disk cannot be repaired.

The problem with installing on another drive is that you need an installer and there's no way to access the AppStore without an OS X system. If you have another Mac you can DL the installer from there, orDL it at an Apple Store. Just copy it from Applications folder to an 8GB or larger disk.

When you do install, don't install the eDrive, it's redundant. Which TechTool do you have, the Pro?

@arthur FWIW - I've run many OS X systems from external firewire 800 drives for months at a time and don't even notice the difference.

If your iMac is the one listed in your profile (ie, from 2007) then it will not have "Internet Recovery" as an option, so if your hard drive is nonfunctional then any recovery partition or other boot partition on it will not work. However, your system does have an optical drive and did come with gray OS X installation DVDs. Additionally, you might have a more recent retail purchase of OS X Lion or Mountain Lion. Any of these can be used as boot discs, so insert them into the optical drive and start the system with the C key held, and when the OS X installer loads you can then attach your external drive and use that as a destination for installing OS X.

Good catch!

You can install Mac OS X on USB-Drive. Go to Disk Utility, click on USB-Drive and select Restore. There is option for Source and Destination drive. Source will be your installation DVD and destination USB drive.

I thought Apple does not make a ML installer DVD.

Good point! I forgot about that. Well than you have only 2 options:

- Install some older OS X on the USB

- Use this app: LionDiskMaker

They don't. What I was talking about was DLing ML and then saving the file to disk. I've done this many times by saving the "Install Mac OS X" downloaded to /Appications before installing. After you install, it self destructs. You can also use Lion Disk maker to create an install DVD.

Well if you cannot see the drive chances are its damaged beyond repair. You can take it out of and replace yourself if you do not have waranty anymore. iFixit.com is a great place where you can find instructions how to do it.

After taking the drive out you can try to salvage it by connecting it to some external case.

That hard drives are dying so often may be connected with your working habits. If you are carrying computer often with you, you should consider to switch to SSD. They are not so shock sensitive as the normal hard drives.