Thomas Bolingbroke wrote:
Is this sustainable? Using my FW drive as my 'main' drive?
On a Late 2009 iMac it sure is, as long as the internal drive still works to send sensor data to OS X to tone down the fans.
And more unfortunately, when the iMac gets no sensor information via that cable, the heat exhaust fans kick into permanent high gear, so that cable must be connected.
When the drive dies you'll have to take the machine to Apple for a drive replacement (read the links why), however if you wait too much longer Apple throws up their hands and say they don't have compatible drives for your machine and suggest you buy a new iMac instead.
It's because Apple tends to carry parts for more recent machines and not older ones, so ask them first.
Your iMac has SATA2 at 3Gb/s, newer drives and computers are SATA3 at 6Gb/s.
So you can continue like you are and place all the wear and tear on the external drive clones, this way the internal drive will last considerably longer "talking" to calm down the fans. Also the clones will make it easy to move to a new machine when the Late 2009 iMac does finally die.
Your machine is 4 years old, it might last another 1-2 years before something else goes on it, like the video card for instance.
I don't see a problem running just like you are until the internal drive fails and forces your hand, by then you would likely consider buying a new machine instead of wasting money having that one repaired by Apple and their outrageous fees.