Can my iMac boot from an external disk?

I'm just afraid that if the computer is already 6 years old, that I'm just living on borrowed time with that method. Will it really make my computer last that much longer? The iMac is already the second oldest iMac version to receive the Mac OS Sierra update.

So it'll be even slower than using the internal HDD?

Ok, seems how it won't hurt to try... I already have a 32 gb flash drive that I can use to test how it will work. If it works decently, I'll go out and purchase the 128 Gb flash drive, if not. I'll install an ssd as you advise.

I will certainly report back one way or another, although hopefully it will work, I know logically I'm siding with you.

Ok, so here's the rundown. I have El Capitan currently installed on my flash drive and I"m running the OS off of it right now. A couple things I have noticed is that, yes, it is a tad slower than the internal 500 GB HDD, and it does seem to lag here and there due to it being bottled down to USB 2.0 speeds. After reading many other forums of people in the same boat, most have decided to risk it and install an SSD inside the machine due to USB 2.0 limiting the speed of an external SSD. However, there are a few out there, and I will provide links below, that made the jump and decided to use their iMac Firewire 800 port and hook up an external SSD through the port as their boot disk. I don't know if it's just their luck or if it's truly faster than the internal HDD. I did find some tests regarding a firewire 800 ssd v. internal HDD...

Internal drive, a WD2500JS:

Code:

Disk Test 68.04
Sequential 93.78
Uncached Write 90.39 55.50 MB/sec [4K blocks]
Uncached Write 95.53 54.05 MB/sec [256K blocks]
Uncached Read 81.87 23.96 MB/sec [4K blocks]
Uncached Read 112.27 56.42 MB/sec [256K blocks]
Random 53.39
Uncached Write 20.74 2.20 MB/sec [4K blocks]
Uncached Write 110.38 35.34 MB/sec [256K blocks]
Uncached Read 97.04 0.69 MB/sec [4K blocks]
Uncached Read 136.24 25.28 MB/sec [256K blocks]

SSD in enclosure connected over FW800:

Code:

Disk Test 130.44
Sequential 87.27
Uncached Write 114.65 70.40 MB/sec [4K blocks]
Uncached Write 108.10 61.16 MB/sec [256K blocks]
Uncached Read 47.90 14.02 MB/sec [4K blocks]
Uncached Read 143.18 71.96 MB/sec [256K blocks]
Random 258.08
Uncached Write 142.01 15.03 MB/sec [4K blocks]
Uncached Write 197.96 63.38 MB/sec [256K blocks]
Uncached Read 1351.91 9.58 MB/sec [4K blocks]
Uncached Read 375.07 69.60 MB/sec [256K blocks]

http://apple.stackexchange.com/questions/19402/is-upgrading-an-imac-to-an-ssd-ov er-usb-2-firewire-an-upgrade

(This guy did it on his 06/07 iMac and it made a difference, but I don't know if it's 5400 Rpm or 7200 Rpm internally)

http://forums.macrumors.com/threads/ssd-drive-worth-it-as-an-external.1364215/

(Post #14 by thestickman states his positive experience from Firewire 800, but he doesn't specifically say which Mac he has, although the forum was originally about a 2011 iMac)

External SSD HD as system drive

(Apple support community post with the solving response have a successful try with Firewire)

http://www.macworld.com/article/2907125/use-an-external-ssd-to-make-an-old-mac-f eel-new-without-cracking-it-open.html

(Lastly, this guy had a Mac Mini that was slowing down and firewire revived it)

I think in our case, we're better off, or at least I am, in just saving the money by using the internal 7200 RPM HDD because it uses a SATA connection which uses the Hard Drive's full potential and is faster than Firewire 800. There might be a few things that are faster using Firewire 800 because it's connecting an SSD, but overall, it's not worth the money to purchase the SSD and an external closure. It's almost $100 for a basic 250 GB SSD and enclosure, and in your case, $240. That kind of money is roughly 15 to 20 percent of a new iMac! I know, it sounds good to say that the Mac is running on an SSD, but in reality the Firewire 800 cuts down on it's ability too much. However, if you do end up going that route, please post on this forum and let us know how the experience went regardless of the outcome.