Upgrading hdd in a fusion drive on iMac

With Big Sur (I think this started back with Catalina), what you see with the collection of different volumes and containers is the "new normal." It's actually very organized and everything is there for a reason. Your files are all in Macintosh HD - Data but when you are using your Mac, the OS makes it look to you like they are on Something called Macintosh HD, like it always used to me.

The actual Macintosh HD holds the operating system now, in a snapshot of that name.

I wouldn't worry about any of those details, but what you describe sounds normal.

As for upgrading the HDD, I would give this some thought because a 2015 iMac is probably nearing end of life. That said, I am using one that seems healthy still, and until a few months ago had been using a 2008 iMac but its power supply failed and that was that. I would replace the fusion drive with the largest internal SSD you can afford. That's what I did with my 2015 iMac, I'm very pleased with it. For this, I would have an Apple Authorized Service Provider do the work, I think for most people it is not straightforward.

A different approach would be to leave the fusion drive where it is and use it for storage or for alternate MacOS boots, and instead follow Jack19's instructions here Use an external SSD as your startup disk … - Apple Community which explains how you can set up an external SSD to use as your main boot drive. One attractive thing about this approach is that no surgery on the iMac is needed, and many commercial external SSDs can be used for this. The additional storage from your original fusion drive is a bonus too.

oldmacbookperson wrote:

These are the read/write speeds:

NEW SSD(Samsung 870 Evo):

-read 560mbps

-write 530mbps

ORIGINAL SSD BLADE:

-read 650mbps

-write 320mbps

This would mean that installing macos on just the new ssd would give me a faster write speed but a slower read speed. I think i might still use the ssd's fusion drive because read speed is pretty important and i will use the 128gb ssd for less "twice" read/write.

I think you are overworrying this for a 2015 iMac, which is obsolete and won't likely last more than a few years more. You will never be able to tell the difference between 560 and 650 MB/s (megabytes, not megabits). I wouldn't worry about the blade, whether it's 24 GB or 128 GB it's not enough storage to worry about, although I guess 128 GB might be a small nice extra storage.

In case you haven't noticed, the fusion drive isn't offered anymore. It did not live up to its billing and created extra ways for things to fail. Look at the number of posts in Apple Discussions about fusion drives that have separated and need to be re-fused. I would not return to that architecture.

I would take the new 1 TB drive to an Apple Authorized Service Provider and have them replace the fusion drive with that and boot from the 1 TB. Or follow the Jack19 link I sent you and boot from an external SSD.

I think I'd skip the Fusioning of the new SSD, Fusion loads stuff from the Main drive to the little SSD part, then unloads it & lods new stuff if full, so it's basically an uneeded step to Fuse two SSDs.

I have a 2015 iMac and replaced its original internal drive with an SSD that is rated at 500-600 MB/s. I'm very pleased with it. Also, as I indicated before, Jack19's link shows how you can use an external SSD if you don't want to go to the extra effort of replacing the internal. The labor cost of replacing my old internal drive with the new internal SSD was about $75, maybe less. The 2015 iMacs require some skill to work on so I would also recommend that.

Hard Drive Install Video

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https://eshop.macsales.com/installvideos/imac_27in5k_14late_hdd/iMac17-1/

I thought so too, but I'll take OWC's word over mine...

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for upgrading the Hard Drive in 27-inch iMac (Late 2012 - Early 2019) models

https://eshop.macsales.com/item/OWC/DIDIMACHDD12/

I'd ignore it, or if you use Apps that you can choose a Scratch Disk, that'd be a good use.

I believe the 128 GB one is faster than the 24 GB one, but still I can't figure out reading & writing data twice would help, though one would thing there'd be a lot less of the "twice" with a 128 GB/1 TB Fusion option.

But setup as fusion it'll read from the 128 GB to see if what is needed is already on the 128 gB, then if not... write what is on the 128 GB back to the 1 TB, then read what is needed from the 1 TB, write it to the 128 GB, then load it into RAM.:)