Erasing HD - Can't unmount disk

When you run Disk Utility check the SMART status of the drive, if there are errors you will need a new disk drive.

If the SMART status is good you could try this.

Have you an external disk you can install Mavericks, have the external disk connected and start the system with, I presume you use CMD r, partition and erase the external disk with a GUID partition and Mac OS extended journal file system.

You should be able to select the external disk to install to with Mavericks installer.

If the computer boots from the external disk try again partitioning and erasing the internal disk.

If you can get a good internal disk with this procedure you can just clone the external drive to the internal with Super Duper or Carbon a Copy Cloner.

I think the machine requires the recovery volume on the disk so you can download and run that software from a Apple to put that back in the internal drive.

It is a bit drawn out, maybe someone can suggest something easier.

Take a look at this one. It combines some of the features of an SSD with a traditional drive. It's much faster and doesn't cost much more.

http://www.amazon.com/Seagate-Solid-Hybrid-2-5-Inch-ST1000LM014/dp/B00B99JUBQ/re f=sr_1_1?ie=UTF8&qid=1396552367&sr=8-1&keywords=seagate+hybrid+1tb

Sorry some more items for to consider.

How are you going to install the OS, I thought the installer was on the faulty hard drive, if you change the drive out you will not have that facility.

You may require the original DVD install set, you can get them for a handling charge from Apple. I don't know which version of OS X that would be. You will require at least SnowLeopard 10.6.8 to update to Mavericks.

Don't install Clean My Mac. That'll cause more problems than it solves.

Pretty much any drive will work. The hybrid I mentioned just gives a lot more speed for a fraction of the cost of an SSD.

Are you on Mavericks now? The easy way to do this would be to put your new drive in an external enclosure. Boot to your Recovery partition by holding the Command and R keys at startup. Partition and format your new drive as a GUID partition, Mac OS X Extended (Journaled) NOT case sensitive as the format. When that's done, install the OS, also via the Recovery partition. Now swap the new drive for the old one.

Put the old drive in the enclosure and see if you can read it if there's anything you need on it.

http://www.amazon.com/Sabrent-2-5-Inch-Aluminum-Enclosure-EC-TB4P/dp/B005EIGUD4/ ref=sr_1_3?ie=UTF8&qid=1396554345&sr=8-3&keywords=2.5+enclosure

You can buy these at pretty much any computer store.

If you can boot into recovery mode I would use Bobs method.

I assumed she had been. Otherwise, trying to format the startup disk is the problem. You can't format or partition the disk you booted from.

The drive as been flaky from all the posts on this thread and a previous thread. If the internal can still boot I agree with you, if not this is another article on how to make a boot drive.

http://osxdaily.com/2013/06/12/make-boot-os-x-mavericks-usb-install-drive/

One other thing to consider... the drive may be ok and it's the SATA cable that's flakey. That happens pretty often. A good way to test this is to get the external enclosure I mentioned in the previous post and move the internal drive to it. If it runs fine from there, the SATA cable is the problem. That's also something you can fix yourself if it turns out to be the problem.

The external enclosure plugs into a USB port, so you do need to plug into USB. The upside is when you're done you can either put in your old drive if it's reliable or just buy another bare drive and you'll have an external for backup or whatever.

There's really no way to check the SATA cable other than what I said, so you need the enclosure.