Troubled Mojave upgrade on 3TB iMac
macOS Mojave installed fine on our MacBooks, but has been a problem on my iMac (27– inch, Late 2013) with a 3TB Fusion Drive. All have FileVault turned on, but the iMac keeps crashing when trying to start up. What can I do?
27–inch iMac models with 3TB hard disks have proved a particular problem with the Mojave upgrade. Those with Boot Camp partitions have been worst affected, and Apple has warned that they need that partition to be removed using Boot Camp Assistant before they’ll upgrade successfully — Apple has detailed this at bit.ly/mojave3tb.
FileVault encryption has also apparently caused some problems, because the version of APFS (the file system) in Mojave is different and not entirely compatible with High Sierra’s.
In many cases, the best plan if the upgrade doesn’t work first time is to start up in macOS Recovery (see bit.ly/mfrcvtm) or from a bootable USB install disk (see bit.ly/instdisk), initialize the Fusion Drive and perform a clean install. Only try to migrate from your Time Machine or another backup once that has worked properly. For that, you must have at least one thorough and reliable backup, or you could lose documents.
Many have reported that performance seems very poor immediately after installing the upgrade, and seeing the spinning beach ball frequently. It’s worth leaving that Mac running and awake for several hours if that’s the case, as some installations require plenty of time for the kernel and file system to settle down before they start to perform more normally. In extreme cases, that may require as long as 12 hours.