Not a RW question - using an External hard to run system

someone suggested that I use an external usb hard drive to run the OS system instead of replacing in internal one - which have been failing over the last 12 months really. Ive run tests and it comes back hard drive failing stuff !
It’s a 2014 iMac 21.5, any pros or cons I should know about ?
Why do it , for two reasons , cost and err cost. Im going to have to get a second hand iMac 21.5 anyway to use at home , so the idea of being able to take the hard drive and carry on working is appealing.

It’ll be really really slow, running even from an external SSD, let along a HDD, connected with USB.

Pulling the screen on that machine isn’t that hard, just use the correct tools, be gentle and take your time.

Or, you should be able to get a local repair shops to do it, cost will depend on your location, but when I looked into it for a newer glued in machine they wanted £90 for the work, including new glue kit, plus the cost of the hardware I wanted installed. I’m happy enough doing it myself, done a few, but I was curious how much they’d charge.

I’ve used an external SSD (usb 3) for booting and running OS betas and it works fine. The speed really wasn’t bad. Booting was probably faster than an internal HDD but slower than my built in SSD.

I also have tried using a regular HDD USB 3 for betas as well and the speed is extremely slow. You wouldn’t want to use it for your regular day to day machine.

Does that machine have USB3 though? I thought not.

USB 3.0 has been around for some time. I was using a 2012 Mac mini with this drive:

The price is a good bit less now than what I paid. I did this for a quick beta of Mohave last year. I got the drive to move my iTunes library onto so I don’t have it for testing any longer.

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I bought a fast Samsung SSD recently, wanted to try the new iMac without the Fusion. Connected via USB3.1 over USBC. Boot time from the internal Fusion was 23sec, from a cloned version of the Fusion on the external 1:38.

I thought it’d be way faster than that.

Boot times are definitely slower over USB. I don’t know for sure why that is but once you’ve booted they run fine. I’ve run BlackMagic speed-test and my SSD USB 3.0 on a Late 2012 Mac mini ran about 3 times faster than the built-in 5400rpm drive.
I’ve since then added a SSD internal drive(same brand) to that same Mac mini and that drive benchmarked about twice as fast as the USB drive did.

I used to have another Mac mini (also late 2012) as well with the original Fusion drive (128 gig SSD) and about the only thing it did noticeably faster was boot. Once I got the SSD in the one mac mini I sold the Fusion drive one. I was very disappointed in the Fusion drive, hopefully they’ve gotten better.

I also have a 2013 MacPro (the cylinder one) and the flash drive (SSD) benchmarks at almost twice the speed of the mac mini with the SSD.

I don’t reboot very often, sometimes I go a month or more , so boot speed has never been an important thing to me. Once I’ve booted then drive speed is important.

Interesting. I was only using boot times as a general indication of speed, never occurred to me that while boot times may be slow operation times might be quick.

I’ll test some more.

A good instructional video here. I’ve just replace the HDD in my 2010 iMac, a bit easier as the glass is held on with magnets rather than being glued in.

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