IMac 5k 27” 2015 w/fusion HDD > SSD replacement

[honeycombz]

Hi, my iMac with Catalina runs slow with 16g ram because it still has HDD w/fusion the process of replacing HDD seems a bit cumbersome for me and I wanted to update to Big Sur and then Monterey. Wondering if anyone knows if I can run a boot SSD externally from the USB3 and if that would provide some performance gains?

 

[Fishrrman]

Of course you can boot and run from an external SSD.
Many, many other users do it this way.

You would need a USB3 SSD, of sufficient size.
500gb or 1tb would do fine. Even a 256gb if you can scrape one up really cheaply.

You have a couple of ways to go:
1. Buy a "ready to use" external USB3 SSD, such as the Samsung t5 or t7.
or...
2. Buy a "bare" SSD (it could be either a "SATA" SSD or an nvme "blade" SSD), and an enclosure to fit (what type of enclosure depends on what form factor of SSD you get.

Then...
Use disk utility to format the SSD to APFS, GUID partition format.

Then...
Either "start from scratch" (install OS, accounts, apps manually)
or...
Use either CarbonCopyCloner or SuperDuper to "clone" the contents of the internal fusion drive to the SSD.

Finally, go to the startup disk preference pane, and set the external SSD to be "the new boot drive".

BE AWARE that with a 2015 iMac, you will see read speeds of around 410-430MBps from an external SSD.

Just wondering... what kind of read/write speeds are you getting NOW (from the fusion drive)?
Use the free Blackmagic Speed Test app to find out.

 

[rpmurray]

I would stay away from USB and go with Thunderbolt. USB drives have a nasty tendency to unexpectedly disconnect when using Apple's more recent OS versions. If money is no object I'd get:

1. The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter. It's the only one that is bi-diectional so will work with your Thiunderbolt 2 ports.
2. The OWC Express 4M2. You need either a powered hub, if you use a bus powered external drive, or an external enclosure that provides power to drives you can install in it. This is the latter. The adapter does not provide power from the iMac to bus powered devices. I'd get the DIY enclosure because while the read speeds for OWC NVMe drives are OK the write speeds are anemic. So if you don't go for any of their offerings with an NVMe built in you'll need:
3. Any NVMe drive that suits your budget. I recommend the Samsungs. I'd go with the Pros, but if that's too expensive you can use the EVOs.

The iMac should fly with this setup. Not as fast as it would be if the ports were Thunderbolt 3, but much faster than what you have now.

 

[Fishrrman]

Thunderbolt won't be any faster than USB3.
In fact, USB3 might even be faster...

 

[rpmurray]

Thunderbolt won't be any faster than USB3.
In fact, USB3 might even be faster...

Maybe, depends on the version of USB 3, and I don't think the 3.0 in this iMac goes faster than 5Gbps whereas Thunderbolt 2 can do 20Gbps. In my experience USB3 tends to be more finicky and susceptible to EMI. And all the stories I've been reading on MR about the unexpected disconnects with USB have me relegating it to low requirement peripherals like mice, keyboards and printers.

 

[Grubster]

Let me know if you do it. I have the same machine, 2015 retina with 1TB fusion drive. I guess the worst thing you can do is try it, leave the fusion drive alone, boot for a bit on the external drive, like the Samsung t7 and see what happens. I've noticed the largest issue with my iMac fusion is the SLOW boot time. But after it boots and everything loads, it runs great. I rarely reboot, I run 2 external monitors, work on Teams, Mail, iMessage, Safari and Citrix workspace app all day long and it doesn't skip a beat. Since I work from home, I am a bit nervous to trust a cheap external SSD plugged in via USB to be stable.

I have 2 2017 MacBook pro's, 13", they boot EXTREMELY fast, and work great, but they aren't full time work computers for me, more part time machines for working outside my home office.