Internal HD failing, using External HD as main drive(music production purposes)

Sure, you can do it quite easily. Simply install OS X on the drive, then boot off of it (hold down option while booting to select the external drive to boot off of). I've successfully booted a Mac Pro using my laptop's hard drive connected via target disk mode before, and this is basically the same process.

It'll be somewhat slower than using the internal drive, but using FireWire instead of USB will ameliorate this somewhat. Just make sure you don't disconnect the drive from the system (or the power from the drive!) while it's running or you'll crash pretty much instantly.

Yup, should not be a problem.

Hook up the drive, boot off the install CD, start Disk Utility to partition/format the external drive with a GUID partition to make it bootable. Then go on to install the OS.

We have one iMac here with no OS on the internal and the OS is installed on 3 different external drives. Each person has their own drive, they just plug it in and boot it up.

louie23 said:

Thank you for the reply That's great to hear! How about the speed of the OS installed in the external drives? Does it give you guys a slow speed at all when it comes to using programs installed on it?

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Is it a USB HDD? If so, it will be okay, but you will definitely see a slow down during startup and starting of applications and big files, since you are limited to 35 MB/s sequential read/write speeds.

louie23 said:

Hmmm, I'm planning to get a firewire one. Assuming I would be getting a USB HD, how slow will it go during startup or using applications? Is it really slow like I would get the spinning rainbow wheel a lot?

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Go with the firewire one, it's faster.

"Is it possible to use an external HD as my main drive and have the iMac boot from there and bypass my internal one? Would I encounter any speed issues with this?"

Just a word of caution here:

I have a 2007 "white Intel" iMac (the last of the white iMacs).

While it will boot from -older- firewire 400 drives, it WILL NOT BOOT from recent (past 2-3 years) firewire 800 external drives. At least, it won't boot from my Other World Computing "Voyager" USB3/firewire800 external "dock". (Aside: the iMac boots up just fine from the dock when using the USB3 port).

Perhaps others will jump in with an explanation, but in the past few years SOMETHING got changed (if not by Apple, then by the makers of external firewire controller chips or boards) that renders "older" Macs un-bootable from "newer" firewire external sources.

This may not be true for every drive, but I have read of other users having the same problems.

My suggestion:
If you decide to "go external", keep the above anecdotal info in mind. If you already have an external firewire device that can boot your older iMac, by all means keep using it. But be aware that if you buy a -new- firewire external device, that it may not be capable of booting the older iMac via firewire.

I would suggest that any newer external device you buy (with boot purposes intended) have BOTH firewire AND USB connectivity, or else you may be disappointed with the results.

Also, I would strongly suggest that ANY USB device you buy be USB3 and not USB2. By the end of this year, USB3 will be the new standard on Macs. And sooner or later, you are going to end up with a new Mac with USB3, which will be able to accommodate the significantly faster speeds that USB3 will offer....