Bootable External SSD For 2017 IMAC

Both 21.5-inch 2017 iMacs also have two Thunderbolt 3 ports that work with USB-C storage devices.

2017 21.5-inch 4K: https://support.apple.com/kb/SP759?viewlocale=en_US&locale=en_US

2017 21.5-inch: https://support.apple.com/kb/SP758?viewlocale=en_US&locale=en_US

The main concern are is the USB enclosure be rated at USB3 speeds min (USB 2 will make your problems worse) and the SSD inside must be rated at 6GB/sec SATA speed. Otherwise you are wasting your money.

I prefer OWC or Crucial SSDs for Macs. They seem to have far fewer issues reported here. Not sure about Crucial but OWC has excellent tech and warranty support.

Since your 2017 iMac supports it, use the USB-C to USB-C cable. You may need to reformat it.

REFORMAT:

1. Go to Disk Utility (Finder > Go > Utilities > Disk Utility)
2. In Disk Utility, select "View" in the top toolbar and select "Show All Devices."
3. In the EXTERNAL section of Disk Utility, look for the top, non-indented, drive. Its name will end in "Media." 
4. Click Erase. Pick a name, use APFS, and GUID Partition Map.
5. Click Erase

Use CCC to clone the drives, then attempt to boot from it.

Cheers,

Jack

Hi,

It depends on which 2017 iMac you are using, 27inch Retina display iMac or 21.5inch one.

If it's a 27inch, you can connect ordinal 2.5inch USB3.0 external enclosure with USB3.0 cable or USB-C one with USB-C cable.

If it's a 21.5inch you need to get USB-C to USB3.0 adapter with USB3.0 cable or USB-C external enclosure with USB-C cable.

From Macworld.com: "Boot your machine with the Option key down to invoke the Startup Manager, then hold down the Control key prior to clicking on a volume to boot. (You may have to use the left and right arrow keys to select a volume first.)".

Cheers,

Jack

Hi,

I think Samsung T7 is suited your purpose.

Get an OWC solution (www.macsales.com), far fewer issues than Samsung. They specialize in Macs and you know their stuff will just work right out of the box and continue to work.