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I was messing around with my iMac and got to a point where I had to reinstall the OS, but it turned out the Fusion Drive had split into a 1 TB hard drive and a 24 GB flash drive. After looking around I found that you can unsplit the Fusion Drive with a Terminal Command in Recovery Mode but that would involve reinstalling macOS afterwards.

There doesn't seem to be a way to fix the split fusion drive without losing my data (or rather, having to restore it from a backup which in my experience is not only a tedious process but also doesn't restore everything...). So I was wondering if you guys think it would be worth it to fix the fusion drive and reinstall the OS or if a split fusion drive is not necessarily a problem.

What are the drawbacks/benefits of having a split fusion drive?

I have a 2017 iMac 27-inch currently running macOS 10.15.6 Beta.

  • Can we assume split fusion drive means you have macOS Catalina with one hard drive and one SSD drive - both OEM and internal? – bmike Jun 20 at 18:33
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    @bmike — yep, should have specified. – Kabir Gupta 10 hours ago
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What are the drawbacks/benefits of having a split fusion drive?

I think the first thing is to define what the purpose of a fusion drive is. A fusion drive was created to so you could get the performance benefits of an SSD while simultaneously getting the value of and capacity of a spinning magnetic drive.

“Fused together” you get a single virtual drive out of two very different physical drives - a small, but fast solid state drive and a slow, but large magnetic drive. Compared to a single solid state drive, it’s nowhere as fast but then again it’s nowhere as slow as a magnetic drive, but it is convenient because it manages where data resides - commonly used files are stored on the SSD and less commonly used files on the magnetic one.

Splitting it, you end up manually managing what goes where, but now the SSD is unburdened by the slowness of the spinning media. You will get a speed increase of the files you put on it. As far as files you “archive away,” they get to reside on the slower magnetic one, but since they’re rarely accessed, speed is not of a concern.

This has been tried countless times before Fusion dives existed. Everyone that I have known that tried to install an OS on a smaller, faster drive and the data, home directories, and applications stored on a larger, slower drive have abandoned this because it ended up being a royal PITA.

Go "All In"

IMO, “messing around with it” is a waste of time because you end up with less than what you started with. If you’re going undertake reconfiguring your fusion drive, go “all in”. Open the iMac and swap out the spinning media for an SSD. If you really want to step things up, change out that smaller PCIe SSD for a larger one as well. Depending on your budget, you could have an 8TB (or more) Fusion drive; or you could have a super fast 4TB drive in RAID 0; or a data secure RAID 1 - all software RAID of course. You could also have a super fast, internal Time Machine backup drive. Now, you've got options.

TL;DR

Unless you're going to open up your iMac and make a significant change like swapping out the 3.5" HDD for an SSD, there's no benefit to tweaking around with your existing Fusion Drive because in it's factory default configuration, you're going to get the best performance in terms of speed and usage as well as value for the money possible.

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    +1 Manually managing separate drives is, indeed, a royal PITA. – user3247189 Jun 21 at 4:17
  • I've been manually managing separate SSD and HDD drives on Windows for the past 10 years or so. It's not even a hassle, much less a PITA. I can't imagine that it would be harder on the Mac; given my experience with macOS, it stands to reason that it would be even easier. Not only the Unix underpinnings enabling powerful things like creating symbolic links, but also the general macOS ease-of-use. I haven't personally managed two separate drives on the Mac because my MacBook only has an SSD. But way back in the pre-OS X days, I always kept a separate boot and data partition. Trivial. – Cody Gray Jun 21 at 7:02
  • Where it's going to get difficult is simply that the asker's SSD is so small (only 24 GB). You can't put the system files there, or even any reasonable portion of them. This small of a drive is almost completely useless for anything but a temporary scratch space for, e.g., unzipping files, storing copies of documents you were currently working on, etc. I imagine that the average user is not going to see real performance benefits to that. (Maybe if you were editing photos?) So because of that size limitation, I agree Fusion Drive's automatic management is the way to go. – Cody Gray Jun 21 at 7:05
  • This seems to be answering why it's not a good idea to split a drive, but the question was if it's a good idea to unsplit one. – Adam Millerchip Jun 22 at 16:19
  • The question was What are the drawbacks/benefits of having a split fusion drive? If you read the answer @AdamMillerchip, I cover both drawbacks and benefits. There’s just way more drawbacks. It’s reality and not everything falls neatly and evenly into a pro/con decision table. – Allan Jun 22 at 16:57
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If you split the parts of a Fusion Drive, the drives are left in a meaningless state. You cannot re-fuse the drive and recover your data.

Restoring a complete snapshot should restore everything, assuming everything has been backed up.

The drawbacks of having a split Fusion drive are that the 24 GB SSD is too small to be an effective system volume, and the 1TB hard drive is as slow as ... well, a hard drive.

From personal experience, I've found that the performance diminishes as the Fusion drive gets more full. I eventually split my Fusion, and replaced the SSD with a 1TB SSD drive, which was noticeably faster than the Fusion. That might be harder to do in an iMac, but you can always run an external drive from Thunderbolt or even USB 3 at a reasonable speed.

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Yes - if your goal is to get the most speed out of your hardware, I would make a backup and let macOS rebuild a fusion drive. It’s way faster that way than alone and less fuss.

The only upside of keeping things split is you can learn how to do storage management manually, learn and then run benchmarks. If that’s your goal - absolutely let things be split and try to see small installs, different filesystem layouts, but I’m assuming you are asking how best to use what you have as opposed to make changes or stay split.

There’s no downside to not using the fusion drive other than losing speed and having more drives show up.

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This happened to me after a botched OS upgrade failed to upgrade the filesystem to APFS.

I did the unsplit, recovered from a time machine backup (taken before the OS upgrade), and everything was back to normal.

If you have a time machine backup and just want to restore to the state before the drive was split, unsplitting and recovering from backup is definitely worthwhile.

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