Can my iMac boot from an external disk?

Your iMac can be set to boot from an external drive. Just install OS X on the SSD and set the SSD as your startup disk in System Preferences -> Startup Disk.

Yes it can boot, however the boot speed will be slow due to the connections a 2010 iMac has. Consider taking the computer to a AASP in your area that provides these upgrades. The charge to install the SSD should be marginal and this would be dramatic improvement in performance over an external SSD.

If you don't know of any AASPs in your area, click the following link to locate one. You can then call and ask how much they charge for the service. https://locate.apple.com/country

Although true, I (a while ago) used my 09 MBP from an externally connected SSD, I was happily surprised when it turned out to be faster that way than on the internal 5400 rpm spinner. So I put the SSD inside, where it was very much faster (once I dealt with the SATA cable nonsense)

Slower to boot, but (depending on RAM size) possibly faster in use after boot is done.

CBennett2498 wrote:

Ok, so my boot time would be slow as my iMac has USB 2.0? Also, what it AASP?

Thanks!

If you use the FW800 connection it will be faster than USB 2 but it will still be slow. I recommend using an internal SSD, while you may not want to do it yourself you will find the performance of an internal SSD dramatically quicker than a FW800 connection. Therefore contact a AASP, get some info and then decide if you want to use internal SSD or not.

CBennett2498 wrote:

My iMac is usually always just on sleep mode when I'm not using it, so I'm not concerned about boot time. However, as I'm considering a ram upgrade, would there be a huge difference between 8 and 16 GB of ram with an external SSD? Or is that something that just needs to be solved with testing/trial and error?

The more RAM you have the less time that will be wasted going to the drive to get something, so yes it will be faster with 8GB, and still faster with 16.

CBennett2498 wrote:

Would a 128 GB Flash Drive work as well? I was particularly looking at this one.

http://www.officedepot.com/a/products/844597/PNY-Turbo-30-USB-30-Flash/

Yes, but you would likely want to go for a long walk and some breakfast while waiting for it to access information let alone booting up. In short, nice try but a very poor idea.

But there would be enough time for a really good breakfast 🙂

CBennett2498 wrote:

Ok, so my boot time would be slow as my iMac has USB 2.0? Also, what it AASP?

Thanks!

An AASP is an Apple Authorized Service Provider.

I still think he should take it to an AASP and have a SSD installed. It may cost a few more $ but at least he will be happy with the thing. I think if he goes with an external SSD he will be very disappointed and rue wasting money on buying the enclosure.

I totally agree, I tried it both ways and internally it seems worth the money, externally not so.