Anyone running macOS (Intel) permanently off an external SSD?

If so, I’d just like to know what my options are with regards to the internal drive, a fusion drive on a Mac mini in my case.

Can I format this completely and use it for storage? It still works, it’s just painfully slow in 2021.

Or do I need to retain this with an OS installed in order to keep the system operational?

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Yes, we ran my wife’s 2013 iMac from an external SSD for years. The OS was on the external SSD and we used the internal HD for storage.

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Yep, running off a Samsung T5 for an iMac.
Internal drive formatted and used for storage. No issues at all.

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Yup - an iMac which has been running off a Samsung T5 for about 2.5 years and a Mac mini running MacOS Server, also amusing, for about the same time. No issues at all

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I’d just like to chime in—and hopefully a community of power users needs this sort of reminder less than most people—please back up regularly if you do this. External memory is just as stable as internal memory. But its connection to your machine isn’t. Whether due to some physical movement, or oddities with your cable, port, or controller on the external drive.

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If you have a fusion drive Mac Mini you should be able to easily swap that drive out for an internal SSD and use another one external for backups. The reason not to use the fusion drive anymore is that is likely >5y old and be in the danger zone of failure.

I use older drives to make an extra stationary backup of data and store it offsite. As the drives are no longer in daily use they can still be used for that

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Yes. Did this with a 2014 Mini, rather than replace the internal drive. Worked without issues at all. Was better than the spinning rust!

I did this for years on a 2009 iMac with a Firewire/Sata dock. Currently booting from a Samsung T7 on my 2019 iMac5k with fusion drive. I was using a SanDisk Extreme USB-C SSD (not the faster Extreme Pro), but it had an annoying quirk - not remounting as a bootable disc following a restart. Completely reliable after a full shut down, but not a restart. Weird. The T7 started out great, but quickly degrades to about 60MB/s write speeds. It isn’t heat related. It can be “revived” by reformatting to HFS+, wiping the drive with zeros using dd in Terminal, then restoring from a SuperDuper backup. It then gets back its very fast ~700MB/s write speeds, but that only lasts for a few days. I’m awaiting delivery of an OWC Envoy Extreme Thunderbolt 3 enclosure for a MVNe M.2 SSD, which I am hoping will be both fast and reliable. I’ll try to remember to update after it arrives.

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I too am using a Samsung T5 as my boot drive on a 2017 iMac with a fusion drive. I create a daily bootable backup to the fusion drive using Carbon Copy Cloner.

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Seems like the replies are for non-M1 machines. Can @chrisa update the topic for the chipset? Thanks

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I have a 2014 Mac Mini that was also painfully slow. It has never been my main computer so I didn’t mind but every system update or any other installs would take forever becuase of the slow internal drive. I formatted a Samsung T5 and managed to install and run the system on that instead. I use the internal as storage like others have said and it works great.

The only drawback for me has been that if I have a second external drive (besides the T5) plugged in and restart the Mac Mini, it gives me a blank white screen at startup and is unable to boot. If I remove the second external drive, it starts up like nothing happened and runs great. I made sure the correct boot disk is selected in the settings but maybe I’ve missed something else. I’ve read some others have had the same issue, not sure if it happens to everyone. I don’t feel like digging into the Mini to replace the internal so it’s just something I’m aware of and always make sure to eject and unplug the second external if it needs to reboot. Otherwise zero issues and a MUCH faster machine - it’s been a night and day difference!

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I’ve now received my OWC Envoy Express thunderbolt enclosure. I did have a problem in that the SSD I bought from Amazon was one of two models that aren’t compatible with the OWC enclosure. I’ve asked OWC to update their marketing copy to no longer make the false claim that it works with any drive. The new SSD I got (WD SN 750 1TB) works just fine, and I’m getting about 1300 MB/s write and 1500 MB/s read times in Black Magic Design Disk Speed Test.