My iMac retina 4k, 21.5inch, late 2015, refuses to install Big Sur. No problem downloading, but the installation process fails after about 40 seconds, reboots, fails, reboots....... I gave up after 10 reboots, but found it looping again today having failed to auto update overnight. I have tried starting from Recovery, reinstalled Catalina, run Diagnostics with no error (ADP000), reset NVRAM, all to no avail. Anybody got any clues about this?
iMac 21.5″, macOS 10.15
Looks like you've identified the problem. There's another community thread about it here;
https://discussions.apple.com/thread/252034376?answerId=253913988022&page=1
Apparently Big Sur tries to install exclusively on the SSD part of the Fusion Drive, but during the installation demands > 40GB storage , which is then much reduced afterwards. My Fusion Drive SSD (disk0) only has 24GB:-
diskutil list
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *24.0 GB disk0
1: EFI EFI 314.6 MB disk0s1
2: Apple_APFS Container disk2 23.7 GB disk0s2
/dev/disk1 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1.0 TB disk1
1: EFI EFI 209.7 MB disk1s1
2: Apple_APFS Container disk2 1000.0 GB disk1s2
/dev/disk2 (synthesized):
#: TYPE NAME SIZE IDENTIFIER
0: APFS Container Scheme - +1.0 TB disk2
Physical Stores disk0s2, disk1s2
1: APFS Volume Macintosh HD - Data 290.7 GB disk2s1
2: APFS Volume Preboot 86.8 MB disk2s2
3: APFS Volume Recovery 528.9 MB disk2s3
4: APFS Volume VM 3.2 GB disk2s4
5: APFS Volume Mac Built In 11.2 GB disk2s5
I don't hold out much hope of Apple fixing this for Fusion Drive 2015 iMacs because they're next in line for obsolescence :( I'm staying with Catalina!
Does the computer have any third party antivirus, cleaning or any other "maintenance" apps installed?
Big Sur Install Loop
