Whiskey asked:
"I'm exploring the option of adding an external SSD and booting from that.
Does anyone know if this is possible?"
OF COURSE it's possible.
It's not only "possible",
it's very easy to do.
You won't understand just HOW easy it is, until after you've done it.
And you won't believe how much faster the iMac will be until after you've done it.
It can also be downright "cheap", considering other alternatives.
The fastest, easiest, cheapest way is to buy something like this:
http://www.amazon.com/exec/obidos/ASIN/B00ZTRY532?tag=delt-20
The 240gb size is probably "all you need".
If you wish, pay a little more for the 480gb version, but you may not really need it.
(TIP: if you don't like the SSD on the desk, you may be able to velcro the Sandisk SSD to the back of the iMac, or to its stand.)
Then, you'll need to do this:
1. Plug it into the iMac
2. Initialize the SSD using Disk Utility
3. Install a "fresh copy" of El Capitan onto it.
4. Get the external set up with apps, accounts, and SOME of your data (not ALL of it)
IMPORTANT:
You didn't tell us how much space was used on your current internal HDD.
That would cue us into whether all of "what you have" will fit onto the SSD, or only some of it.
The SSD should have:
- The OS, of course
- Your apps
- Your accounts (home folders), but you may have to leave some things out (see below)
- Possibly some data files
IF you have large libraries of pictures, music, and movies, it's best to leave them on the internal HDD. It's pretty easy to tell apps like iTunes, Photos, etc. to "look at" the internal drive for the library instead of looking on the SSD.
This won't affect the overall speed of the SSD.
When you use "an external booter" like this, be aware that after booting you will now see TWO drive icons on the desktop -- one is the boot SSD, the other is your internal drive.
Give them distinct names and you'll have no trouble choosing between them.
Personal experience:
I've been booting and running my 2012 Mac Mini from an externally-mounted SSD for almost FOUR YEARS now (since the day I unboxed it). It runs today as fast as it did the day that I first set it up. No problems at all.
Again, putting together an external booter is easy.
The most challenging part is deciding what you will install on the SSD, vs what you have to leave behind on the old internal HDD.
