Looking in more detail at newer tricks used by APFS on the data of files: sparse files, which can squeeze vast empty files almost to nothing; file clones; and compression, opening up in Big Sur.
APFS
Disk structure diagrammed for comparison, from the partition table, to HFS+ and APFS volumes, with further references.
What you need to know about the APFS file system, its containers and volumes, so you can understand errors and how to repair them.
What you need to know about the HFS+ file system, so you can understand error reports and how to repair it using Disk Utility and other tools.
What you need to know about disk formatting using the GUID Partition Table (GPT) to understand its problems and how to repair it.
One was just a bit wonky, the other has made a mess of many who installed the update. So what went wrong?
Concentrate on Time Machine backups, scripts, secondary updates, and whether you still have to retain access to 32-bit software. Some hints.
One new tool to list capacities on mounted volumes, the other to deliver detailed information on keychains.
How to store a 400 GB sparse bundle on a volume which only has 200 GB free space – provided that you don’t actually use the space that isn’t there.
Disk Utility can’t image APFS volumes, and normally returns an error if you try to image a container. Here’s the workaround.
The Eclectic Light Company 