How to tell whether someone has been eating the (APFS) porridge on one of your disks, using Get Info and DiskUtility’s First Aid. And a surprise hidden volume from 3 years ago.
APFS
The only disk images of varying size used to be sparse bundles and sparse disk images. Now plain read-write disk images can also vary in the disk space they take, as explained here.
When do sparse files explode to full size, and how could you preserve them in transit? Can you copy clones or snapshots? How to preserve extended attributes?
Focussing on sparse bundles and UDIF read-write disk images, this explains their types, structure, how their size can change, and how fast they are in use.
APFS or HFS+? Which can Time Machine back up to? What about hard disks? Which format for use on PCs? And which are supported by Disk Utility now?
macOS has an elaborate set of rules determining which types of xattr are preserve during different types of copying, including syncing to iCloud Drive. Here they are in full detail.
Extended attributes were added to Mac OS X 10.4, and soon supported the quarantine xattr. They have since flourished, and have valuable properties.
Released in Mac OS X 10.5 Leopard on 26 October 2007, it supported Time Capsules launched in January 2008, and in Big Sur could back up to APFS.
Exactly what is the disk shown in the Finder named Macintosh HD, in Big Sur and later versions of macOS? An exploration of the structure of the modern boot disk.
Symlinks are popular but prove fragile when folders get renamed or moved. Hard links don’t support directories and can’t cross volumes. The alias works best overall, but isn’t supported in Terminal.
The Eclectic Light Company 