Booting from external USB 3.1 SSD

Without a powersupply no it won't boot. I would recommend using a higher quality device than a WD. I'd recommend buying an OWC enclosure and SSD and building your own. These are much higher quality than anything WD has available and they are powered. What I'd look at is something like:

OWC Mercury Elite Pro eSATA, FireWire 400/800 &... at MacSales.com and a

OWC DIY Kit: Data Doubler + 250GB Mercury Electra... at MacSales.com

and then you have a solution.

It would cheaper and more reliable to just replace the HDD in your 2013 iMac, and/or add an SSD if you looking for speed.

see > iMac Intel 27" EMC 2639 Repair - iFixit

I with rkaufmann87... bare WD HDD's are great, but I will not use their failure prone External Passports and MyBooks.

There IS NO problem running a Mac from an external drive!

I have been running my older iMac from an OWC externally enclosed SSD for nearly 9 months, now!

No problems.

I second rkaufmann's suggestion of purchasing a better made, better quality enclosed, external from OWC (macsales.com).

This company is an ALL Mac-centric business with very good quality Mac products and super customer service!

I own two OWC drives for backup drives.

They are a great company to deal with!

Good Luck to You!

It's better to have an external drive, on a desktop computer, use a dedicated, seperate power supply instead of the external drive always drawing/draining extra power from the iMac’s own internal power supply.

In addition larger storage capacity drives really need to be using there own power supply.

Also, most externally enclosed SSDs won't boot just using the iMac's own power supply though a USB cable as USB does not supply enough power needed, most of the time, at boot up to boot a drive to run a Mac from.

In a Thunderbolt connected external hard drive or SSD, using an external drive without the need of an external power supply may work (I have never tested or seen this) as Thunderbolt supplies a lot more power, through the cable, to be able drive an extra monitor (something USB cannot do), so a Thunderbolt connection may carry enough power “umph” to jump start an external drive to boot at startup.

The issue is that Thunderbolt connection enclosed external drives ARE A LOT MORE EXPENSIVE to purchase than their USB 3.0/3.1/-C counterparts and you are already whining about the $10 cost of a dedicated power supply for a USB 3.0/3.1 external SSD drive.

So...

Replacing a drive in a late 2013 27" iMac may look quite daunting

and had me hesitant for a very long time. But after much frustration

of not being able to install High Sierra on an external drive with a

dead internal drive, I bit the bullet and got one of the OWC upgrade

kits with all the tools and special temp sensor cable.

The watched the install video multiple times and had it on my

laptop if needed. The install went smoothly and I am now happily

running macOS High Sierra on an internal SSD on my iMac.

BTW, I have been booting a few years from bus powered external

Thunderbolt attached and USB3 attached SSDs. Portable drives

of any sort boot just fine without external power.

Full size 3.5" HDDs however do require external supplies.

Yes,

You will need the power supply.

I use the additional power supplies on all of my drives.