External SSD for New OS on a Late 2014 iMac

Welcome to the Apple Support Communities.

Yes, you can do that. I would suggest getting a USB external SSD, since Thunderbolt 1/2 cannot be found anymore. A great choice is a G-Technology G-DRIVE mobile SSD R-Series Storage - Apple.

You cannot use Apple's Thunderbolt adapter, because it is for Thunderbolt 3 only (not USB-C), and the Thunderbolt 3 device must provide its own power, and almost all Thunderbolt 3 SSDs are bus powered.

Here's how to put the latest version of macOS on an external SSD:

1. Connect the SSD to the iMac.
2. Shut off the iMac, turn it on, and immediately press and hold "Option" + "Command" + R.
3. Once in macOS Recovery, connect to WiFi in the top right corner. If possible, connect directly to the internet with an Ethernet cable. 
4. Go to Disk Utility.
5. IMPORTANT: Select "View" > "Show All Devices".
6. Select the main SSD drive. It will be the top, non-indented drive in the "External" section. Its name will end in "Media".
7. Click "Erase", and input the following:
8. Name: Could be anything, or a name such as “Macintosh HD”.
9. Format: APFS
10. Scheme: GUID Partition Map
11. Click Erase.
12. Quit Disk Utility.
13. Select "Reinstall macOS".
14. IMPORTANT: Make sure to select the new external SSD as the installation target for macOS. External disks are orange and silver in macOS.
15. Let the installation process complete.
16. During Setup Assistant, you can choose to transfer data from your internal drive or not. Follow the on-screen steps to move your data from the source to the new macOS on the SSD if you wish.

Cheers,

Jack

Give a serious look at the external SSDs offered by OWC (MacSales.com). Contact their customer support personnel to get their recommendation of the optimal drive for you particular iMac model and needs. I'd look for a 1 TB or larger model depending on your budget.

I want to compliment you for thinking this issue through so thoroughly. I see nothing wrong with your idea of running the newer operating system from an external drive. In the past, I have done exactly what you want to do and for the same reason. Here are my additional thoughts. Be sure that the external drive is a solid state drive (SSD), not a slower HD. Get one with sufficient space to meed your storage needs. These days those external drives are cost effective, compact, normally require no external power and very easy to install with vendor provided cables. Just plug into an available slot and use a cable that has the right connector on each end. You should not have a problem. You might have to reformat the drive, though. I assume that you know how to startup from the alternative drive.

That would be a very good choice. However, for an iMac you don't the the portability aspect and this one would be good: OWC Envoy Pro EX 1.0TB USB-C (Thunderbolt 3) and is $10 less expensive.

Either would be very good. The difference is just esthetics.

cwfromtx wrote:

I like this drive but my iMac doesn't have Thunderbolt 3; only TB2 and USB. Could I connect using a USB-A to USB-C cable to this drive. The drive specs say the host has to be USB3.0 capable and I'm not sure my iMac is. Even if that is true; will I get the read/write capability this drive has; namely 917/956 MB/s. The On-The-Go Pro drive is only 444/446 MB/s.

No matter how fast the external SSD is, you will never see more than ~450MB/s due to the older USB connectivity your iMac has. For this reason, the Envoy Pro EX would not work without a different cable (USB-C to USB-A) which is not included and is an additional cost.

If you get the Mercury On-The-Go Pro, get the extra power supply (OWC Power Adapter for Mercury On-The-Go - OWC).

The G-DRIVE SSD I linked earlier is also a great option.

Cheers,

Jack

I can't tell you as I don't know. Call OWC customer support and ask them. They'll know and what adaptor to get.

cwfromtx wrote:

Thanks, Jack. I went to OWC because the 1TB version the G-Drive isn't available for at least a month.

Oh, I see that. Sorry!