Reformat old windows hard drive for use externally I have an old 100 GB hard drive from a windows laptop that I would like to make use of as a portable, external hard drive. I ordered a cheap case on Amazon and it works fine. However, the disk is still unreadable by my iMac. How do I reformat the hard drive (in Disk Utility?) so that I can use it to just store files for me?

When I plugged my hard drive into my computer, a message came up saying that it was unreadable by this operating system. I opened up Disk Utility and I can view it there. 

The main essence of my question is what can I do from an application like Disk Utility that will enable me to use the hard drive for file storage on my mac.

Details on the hard drive:


Case connects to SATA which is an adapter to a USB cable. 
Connected through USB 2.0 to built in USB port on my iMac 10.8.2
Have you opened Disk Utility and tried to format it already? Sometimes a drive will not appear on the desktop, but will appear in Disk Utility.

I've had windows drives show up sometimes, and not other times. What you could do to make it more likely to show up is go to an available (hopefully not too difficult) windows machine and reformat the drive as a FAT/MS-DOS drive (don't do quick-format). This should be easier for the mac to see.

Once you can see the blank FAT formatted drive, open Disk Utility and then select it on the left sidebar and erase it again as a "Mac OS Extended (Journaled)" (HFS+) format.

There are other methods like using an emulation environment like Parallels Desktop (IMHO bad SW company), VMWare Fusion (my choice), or VirtualBox (free) (all of these require a windows install disc); or you could even try using programs like MacFuse or NTFS-3G, Tuxera, etc., to mount windows (NTFS) drives directly in the Finder. But unless a mac you can find has one of these solutions, it's probably not worth it to go through the trouble just to erase one drive.

Lastly, I've also had some external USB enclosures not show up on some versions of OS X! So the cheapy enclosure you got just might not work with your installation anyway?

Details that might be helpful for potential helpers:
HD interface PATA/ATA/IDE or SATA?
OS X Version?
iMac model year or identifier?
Enclosure is assumed to be USB 2.0 high speed plugged into built-in port on iMac. Correct if wrong.