I have a 2019 iMac 21.5" which has a 1TB fusion drive. But the HDD is getting old and slow (and it obviously wasn't fast to begin with).

Until recently, I also had Windows Bootcamp installed on the HDD, and it was horribly slow. To fix this, I bought an external SSD and now I boot Windows 11 on it via EFI. It's so fast, it's actually better than using my Mac partition. So I want to make the Mac side faster by doing a clean install of MacOS Big Sur to the 28GB SSD and then partition the new 480GB SSD: 240GB for Windows 11, and 240GB for Mac applications and commonly accessed data when running MacOS. This leaves the 1TB HDD for storage of files/application I use rarely.

I assume I want to install MacOS on the 28GB SSD because my new SSD has a speed of 600 MBps and is connected via USB 3.0, is that even slower? So I believe the internal one is faster.

However, I have a few things I don't quite understand about how everything is structured on a Mac.

If I was to split my fusion drive, and then install MacOS to the 28GB SSD. How do I tell the Mac I want to use the external 240GB partition for Applications, Library, System, and Users? I assume having these on SSD will speed up my Mac (please correct me if I'm wrong).

I'm also very confused about what the Macintosh HD/System folder actually is and how this fits into what I want to do.

I want to know exactly what I need to do before I attempt this. Any help would be greatly appreciated.

  • Just curious why USB 3.0 (5 GB/s). Why not USB 3.1 Gen 2 (10 Gb/s) or Thunderbolt 3 (up to 40 Gb/s)? Mar 16 at 23:50
  • 1
    I chuckle at the idea of creating a fusion drive using the internal SSD and external SSD. Mar 16 at 23:56
  • @DavidAnderson Hi David. You helped me last week in getting this far, and explained the speeds. Because I wasn't sure whether the drive was my problem (it wasn't but I fixed it anyway), I chose to buy a cheap Kingston drive (6Gb/s) and a cheap USB 3.0 SATA cable (5Gb/s). I can upgrade these, if you think it will help. Are you also suggesting that if I do this, the Mac can boot from the external SSD and be faster than the internal SATA? I will be honest I'm very confused with the numbers. 6Gb/s seems to be the fastest SSDs come, but using 40Gb/s Thunderbolt 3 will still improve it? Mar 17 at 1:38
  • This is a removable drive and by it’s very nature is prone to disconnecting unexpectedly from the host. This will cause all sorts of issues where part of the mounted OS filesystem is located here. I highly advise against this.
    – Allan
    Mar 17 at 7:11


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