Q: Install El Capitan on exterior SSD, prior to SSD upgrade?

Installing on external first then doing the surgery after EC is stable is a very good idea.  That way you do not lose your existing functional system until you are ready to intentionally delete it.

yes, that works. The Mac does not have a fixed "magic place" it boots from.

The Mac will Install on and boot from any appropriate drive. Run the Installer and take care to specify WHERE the software is to be Installed. The last-installed on drive becomes the default, changeable in:

System Preferences > Startup disk

that is also a way you can get a DiskMaker-X Installer thumb drive to be the Startup Disk to start the Install.

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If you are limited to Recovery Mode:

Which version of OS X is installed by OS X Recovery?

• If you use the Recovery System stored on your startup drive to reinstall OS X, it installs the most recent version of OS X previously installed on this computer.
• If you use Internet Recovery to reinstall OS X, it installs the version of OS X that originally came with your computer. After installation is finished, use the Mac App Store to install related updates or later versions of OS X that you have previously purchased.

from:

https://support.apple.com/en-us/HT201314

although the method you describe is slower, it has the advantage that it can use the Recovery on the old drive to download the new version, if you need to do that. It also has the advantage that is separates issues surrounding "drive surgery" from issues of Mac OS X Installation. Since some users have reported problems with Internal drive cables, I prefer to do the Install on an external drive, then do the surgery once the Install is stable and working.

Alternatively, install the new SSD first, remove the HDD and put it in an external enclosure. Boot from the Recovery HD on the external, partition and format the new SSD using Disk Utility, then use the Restore option of Disk Utility to clone the volume to the new SSD.

The way Kappy  is suggesting will be faster, but (in my opinion) has fewer protections against side issues and complications that can make debugging any problems more difficult.

But then again, you may not encounter any problems, so the longer install and test times (of the drive in a s-l-o-w External enclosure) may not be anything but an annoyance.

The risks of speed-response testing in a USB 2.0 enclosure ... indeed.

Swapping drives and installing on the now-internal SSD works just as well, of course.  Still keeps the old install protected.

Hi Van-don:

Have you considered cloning your HDD to the SSD in an external case http://eshop.macsales.com/Search/Search.cfm?Ntk=Primary&Ns=P_Price%7c0&Ne=5000&N =6430&Ntt=On-The-Go+Kit by using either Carbon Copy Cloner https://bombich.com/ or Super Duper.

This will make a bootable clone which you can test.

This is how I upgraded my Early 2011 Macbook Pro. I have a Samsung 850 EVO 500 GB SSD and it works good.

Also when you install the SSD, you are using El Capitan which support "Trim".

You will want to enable Trim.

To enable TRIM, first save all your open documents as this will reboot your Mac.

1) launch Terminal.

2) Type "sudo trimforce enable".

3) Confirm that you want to enable trim.

4) Reboot your Mac.

To verify that TRIM is enabled, click the Apple icon in the top left of your screen. Choose "About this Mac" -> "System Report" -> "SATA/SATA Express" and look for the entry "Trim Support". It should say "yes" if trim has been enabled.

Kim

TRIM is good, but does not work over USB 2.0 or 3.0.  Maybe not over Firewire.

After the SSD is installed internally, you may need to let the SSD garbage collect.  Using the option+power boot and letting it sit at "waiting to select boot drive" for a few hours can let that occur.  Then whenever the system boots it will start TRIM automatically and the garbage collection will occur with all TRIM engaged.

The EXACT reason is spelled out in scrolling messages on the screen. These messages are also written to the system log.

But the default for Startup is to suppress these messages. If you would like to see them in Real-Time, activate Verbose Mode:

How to start up your Mac in single-user or verbose mode - Apple Support

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Hi Van-don:

You say you have cloned your HDD to the SSD and are currently running it.

Did you replace the HDD or are you running from an external drive.

Try going to System Preferences > Startup Disk and ensure your SSD is selected. If not it still may be looking for your HDD.

Kim

you can make a time Machine back up and restore from it using recovery tools once you install your ssd