2017 iMac 27" external boot SSD questions.

EODMCR wrote:

Hi, I have an old 2017 27" iMac that is really struggling with the internal HD and I wish to boot from an external SSD (either SSD or NVME in an external case).

Q1: What is the best connection to use between external HD and iMac or does it not really matter?

USB3 (external) to USB3 (iMac)

USB3 (external) to Thunderbolt 3 (iMac)

USBC (external) to Thunderbolt 3 (iMac)

Thunderbolt will be the fastest connection, also the most expensive. USB-3 will be plenty fast for that Mac if you're using an SSD of some sort.

Q2: Once I install the OS on the external drive, will it boot from that by default when I power on the iM?

After you install the OS and connect the drive, you'll go to System Preferences > Startup Disk to set the external drive as the boot drive. After that, the Mac will continue to use that drive to startup until you disconnect it or reset it or use the Option key at startup to select another drive.

Q3: I don't plan to immediately format the internal HD, so until then, will I still be able to boot from the Internal HD if I unplug the external one?

Yes, as long as it remains healthy you'll be able to boot from the internal drive.

Thanks in advance

You're welcome. 🙂

Bonus round: OWC - External Drives resource. (I am unaffiliated)

EODMCR wrote:

Brilliant answers, thank you :)

You're welcome. 🙂

My last question is considering the type of ports I have available at the rear of the 2017 iMac. Should I invest in a basic SSD, an M.2 SSD or the faster NVME SSDs?

Obv not your last question. 😉

If you invest in the faster SSD, it will be something you can potentially use after the iMac has exceeded its lifespan and you've moved on.

If I am not mistaken (and please correct me if I am wrong), because of the speeds expected with USB 3 to Thunderbolt 3 cables, I should just go for a M.2 SSD at most?

My understanding of real world speeds via the many different connection protocols is limited, but I don't think you'd go wrong with an M.2 drive on that iMac. I have an OWC M.2 drive on a 2018 Mac mini used for music production that does just fine.

Oh... and one crucial question. Does the Thunderbolt 3 port provide the power to run the external M.2 SSD? I know USB ports generally can

Yes, Thunderbolt 3 / USB-C does provide power to an external device. Some external SSD enclosures can use their own wall ac adapter, which some veteran users would say is better, especially if those enclosures have additional docking features.