How do you boot from an external hard drive if...

Thanks in advance to anyone who can offer any advice. My 24" IMAC suddenly had hard drive ****. After much investigation it has been determined that my internal hard drive has bit the dust. Instead of taking the IMAC apart and replacing the internal drive(which I know a bit about and also could get help doing)can I just replace it with an external drive and load the OS on that and boot from there? If so what exactly, step by step, would I need to do? What are the disadvantages of doing this? If this is possible it seems like an easier solution than replacing the internal drive.
Again, thanks for the help.
Bill

easybpw wrote:
can I just replace it with an external drive and load the OS on that and boot from there?
Yes, but it's not recommended for long-term use.
If so what exactly, step by step, would I need to do?
First, make sure the drive will boot a Mac. Most will, but some won't. If it's a Western Digital, check here: http://wdc.custhelp.com/cgi-bin/wdc.cfg/php/enduser/stdadp.php?pfaqid=1787. But note the disclaimer that they don't support it +*at all.+*
If you're reasonably sure the drive is capable of booting your Mac, you need to format it. Start up your Mac from the Install disc (insert it, and hold the "C" key while starting up). After selecting your language, select Utilities, then +Disk Utility+ from the menubar.
Select the drive, select the top line (with the make and size), click the Partition tab and select 1 Partition from the the pop-up menu under +Volume Scheme,+ unless you need more than one.
Give it a name under +Volume Information,+ and select +Mac OS Extended (Journaled)+ for the Format unless you're certain you need Mac OS Extended (Case-sensitive, journaled). Click the Options button below the box, and select GUID (that should be the default, but make sure), and click the OK button, then Apply. That should only take a few moments. Quit Disk Utility and you’ll see the Utilities menu again.
Quit that and proceed with the installation to the new drive. When done, I think it will reboot automatically from the external; if not, start up while holding down the Option key, which will give you the +Startup Manager,+ where you can select the desired drive.

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