Attended the iMac 27 inch 2014 Fusion Drive system about a little more than month and half ago (128GB Flash storage + 1TB SATA Hard Drive) where client iMac became too slow after updating Big Sur 11.3 beta to 11.4 beta. (Our client joined Apple Developer program and Big Sur was auto updated from 11.3 beta to 11.4 beta at that time) The SATA Hard Drive was about 90% full. Starting up an iMac, you can only see the rainbow colour hour glass going around for too long that it’s almost impossible to do anything.
For whatever the reason, it appeared the internal Fusion link between a fast Apple Flash drive and a very slow 1TB SATA drive was broken. I did not bother fixing the link (waste of time), instead, replaced the very notorious 1TB SATA drive (*eagate) with 2TB 2.5 inch SSD drive and setting up Big Sur 11.23 official release migrating whole data contents of 1TB SATA drive (took more than 12 hours). Problem solved all together. The *eagate SATA drive is so notorious for it’s failure rate (more than 1% – it means one in a hundred will die within a year for no reason) and I do not know why Apple keeps using this drive. (most likely it’s the cheapest drive in the market).
Big Sur compressed installation file size is about 12GB resource hungry monster file. Considering the latest Windows 10 installation iso file (either Pro or Home) still fits on single layer DVD disk (4.3GB) you can see how big the macOS Big Sur installation file is. I would not recommend installing Big Sur on a Fusion Drive system-based Apple computer at all – you are just asking for trouble.
If you really want to continue installing Big Sur on your Fusion Drive system Mac computer, the simplest solution is to replace the internal very slow SATA drive with a SSD first. (If you do, you do not have to worry about Fusion Drive system link is broken or not as the internal replaced SSD is fast enough to cope with the resource hungry Big Sur operating system)
Apple Fusion Drive System (Wiki)
The Fusion Drive was announced as part of an Apple event held on October 23, 2012, with the first supporting products being two desktops: the iMac and Mac Mini with OS X Mountain Lion released in late 2012. Fusion Drive remains available in subsequent models of these computers, but was not expanded to other Apple devices: the recent MacBook and Mac Pro models use exclusively flash storage, and while this was an optional upgrade for the mid-2012 non-Retina MacBook Pro discontinued by Apple, it will replace the standard hard disk drive instead of complementing it in the fashion of Fusion Drive. Supported products have the following configurations:
Apple’s Fusion Drive design incorporates proprietary features with limited documentation. It has been reported that the design of Fusion Drive has been influenced by a research project called Hystor. According to the paper, this hybrid storage system unifies a high-speed SSD and a large-capacity hard drive with several design considerations of which one has been used in the Fusion Drive.
- The SSD and the hard drive are logically merged into a single block device managed by the operating system, which is independent of file systems and requires no changes to applications.
- A portion of SSD space is used as a write-back buffer to absorb incoming write traffic, which hides perceivable latencies and boosts write performance.
- More frequently accessed data is stored on the SSD and the larger, less frequently accessed data stored on the HDD.
- Data movement is based on access patterns: if data has been on the HDD and suddenly becomes frequently accessed, it will usually get moved to the SSD by the program controlling the Fusion Drive. During idle periods, data is adaptively migrated to the most suitable device to provide sustained data processing performance for users.
Several experimental studies have been conducted to speculate about the internal mechanism of Fusion Drive. A number of speculations are available but not completely confirmed.
- Fusion Drive is a block-level solution based on Apple’s Core Storage, a logical volume manager managing multiple physical devices. The capacity of a Fusion Drive is confirmed to be the sum of two devices. Fusion Drive is file system agnostic and effective for both HFS Plus and ZFS
- Part of the SSD space is used as a write buffer for incoming writes. In the stable state, a minimum 4 GB space is reserved for buffering writes. A small spare area is set aside on the SSD for performance consistency.
- Data is promoted to the SSD based on its access frequency. The frequency is detected at the block level]and below file system memory cache. Data migration happens in 128 KB chunks during idle or light I/O periods.
- Operating system and other critical documents are always cached on the SSD. Applications are likely to be handled similarly. A regular file can reside on both devices.
As you can see from the above explanation, the Apple Fusion Drive system is a very cheap solution for Apple using 2nd grade component parts to provide customers for more storage space. Also, the size of fast Apple SSD component is degraded so much from 128GB to the 24GB or 32GB drive. This is unheard of! Whilst Apple computers are a much more expensive than alternative PC computers, I must say Apple did not provide the best solution for users to have affordable reliable Apple computers.
The above image was taken from another broken iMac 27 inch 2017 component
Furthermore, while I love Apple computers, I cannot agree to the company’s very arrogant “Apple Way” policy where the company tries to dictate users what they can and cannot do. Users have to decide what components their favourite Apple computer can have at the time of purchase. There is no upgrade available in the future. In the “Apple Way”, what you buy is what you die with. Fusion Drive system is just one of them. I’m not too sure Apple will explain the full extent of the Pros and Cons of the Fusion Drive system at the time of purchase. There are all sorts of hidden issues using Fusion Drive system. The latest M1 Mac computers no longer have Fusion Drive system, but instead, they become all onboard system. For Apple, it may work for their benefit for quality control purposes, but do they really explain to the users – when M1 Mac dies so as most important data inside? That is the biggest reason why I do not have any plan to purchase M1 Mac computer.
You can refer to the following link to find out why you should avoid using Apple’s Fusion Drive system.
Another client had an issue with the iMac 27-inch Ratina 5K late 2014. My client tried to update macOS Catalina to Big Sur. After 10 hours, it did not finish. Tried Safe Mode to install Big Sur. After the install was finished, everything became very slow and my client could not do anything. My client tried the recovery mode and it corrupted the system and the iMac became unusable.
The fusion drive system was gone and my client could not to do anything. I simply replace the internal notorious conventional 1TB hard drive (*eagate) with a 1TB SSD drive. Use the USB installer USB 3 flash drive with Big Sur 11.5.1 and finished the install in 30 minutes. Two drives now appear in the Disk Utility, but I could not mount Apple’s 128GB SSD. Just erase the Apple 128GB SSD using the APFS format. My client now has a 1TB SSD + 128GB (but because of the actual size you can use, I named it 120GB) extra Apple SSD space.
Again, please do not install Big Sur on Mac computer with fusion drive system.
If you need help for Mac computerr (Sydney CBD, Sydney North Shore – particularly in Epping, Chatswood, Roseville, Lindfield, Killara, Gordon, Pymble, Turramurra, Wahroonga, Warrawee, Hornsby, Macquarie Park and St Ives area) please contact Harry 0417 424 909. Or refer to our Service page.