Time Machine (TM) is Apple’s slick back-up facility. In the usual case, one will back-up (BU) to an external attached hard drive somewhat larger (recommended at least double) in capacity than the data to be backed-up. Time Machine has a long history, but it has been operationally less fussy for the last few years. The following is accurate since Time Capsule was retired in 2018. Also, only Mac backups to an attached drive, or to a shared drive on another Mac via file sharing, are described here; NAS devices are out of this scope. A shared Mac no longer has to run under MacOS Server, as was once required.
The Backup File
Apple uses a Sparse Disk Image Bundle file for backups because its structure supports incremental builds as add-on file ‘bands’. The file is named by TM for the computer on which TM is running, e.g. Big Mac.sparsebundle.
General Operation
To get the backup ball rolling, simply visit the TM preference pane, select the BU drive to use, and turn on TM. If the target is a clean APFS drive, no prep is needed. TM will create Incremental backups occur automatically, hourly for the current 24 hours, then weekly for the current month, then monthly. When the number of backups over time exceeds the available storage, older hourly and daily backups are pruned automatically. Ultimately, a disk full notification may result.
One can tailor the data to be backed-up by excluding folders. Else it all gets backed-up. TM will automatically exclude attached volumes including the BU volume. These can be removed from the exclusion list if they are to be part of a combined backup.
One restores any given file by entering TM via the app, selecting the back-up time of the file status one wants to retrieve, then selecting the restore option. An entire backed-up drive can be restored through the Apple Migration Assistant utility (or via Setup Assistant if initializing a new Mac). Point the utility at a backup file and restored data is written to the new disk.
Note that the command hdiutil can also create an empty sparse disk image bundle if needed. Further, if the initial file ever runs out of room, one can use hdiutil to increase the file size to permit further backup operations.
It is reported that the server need not be a Mac for this all to work, but I have only tried it with Macs.
FWIW, Apple left a back door switch available to enable TM visibility of ‘unsupported’ network drives. By default, this switch is set to off; ro turn it on:
defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1
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