Chosen Solution
Hi! So, I upgraded from High Sierra to Mojave on a late 2013 iMac. I’m having lots of issues with Mojave so I’m want to downgrade back, which I know is possible. However, I’m not sure the best way to go about it if I want to keep all the data. I’m currently making a Time Machine backup, which is running INCREDIBLY slowly, but it will be the only backup I have. I’m assuming that if I were to reinstall from a Time Machine backup in recovery mode, it would reinstall Mojave as the OS. How can I reinstall High Sierra as the OS and then reinstall only my files (and not the OS)? Any help much appreciated.
Performance issues won’t be fixed by down grading the OS, Often with Hard Disks (spinning rust drive) the issue is file fragmentation. The next issue is Fusion Drive’s are not very good if you want performance! They are good for fast boot ups or when you are constantly running the same code over and over. But as the flash cache drive is not very large it doesn’t take long to get bogged down. I would stick with Mojave as I can tell you its not any slower than High Sierra. But, I do agree APFS is not very good on a SATA or mSATA drive your system has. This has to do with the I/O limit of SATA III (6.0 Gb’s) and the queue depth of the I/O channel. Both High Sierra and Mojave use APFS and only if you’ve upgraded a HD will the older HFS+ file system not get upgraded with High Sierra from Sierra. Which is what the Discord group noted the performance changed after upgrading to Mojave as the installer would in fact update the older HFS+ to APFS at that point no matter the older version! So that then leads us to jumping back to Sierra to get back to HFS+. But are we just fighting a problem that could be fixed more easily, by upgrading the RAM to 16GB. But thats not possible in the Late 2013 systems as Apple soldered the RAM! So what to do?? So I would encourage you to make sure your drive is healthy and if it is reformat it with APFS and re-install Mojave first to see if the drives health was the real issue here if you have either a HDD or Fusion Drive. The next option installing a blade SSD if you only have a HDD system and make that your boot drive. I would go with something in the 512GB size. If you have a Fusion Drive I would still replace the Blade SSD with the 512GB and break the Fusion drive set so the blade drive is your boot drive, that is about all you can do and this would offer much more performance than a SATA HDD!
