When the Time Machine drive is full and deletes the oldest stuff to make room, does it take into consideration whether you have only one backup of a file?
Time Machine’s oldest backup is on 1/1/2021, and on that date it backed up files A, B, and C.
As time goes on, only file A is edited, the latest edit occurring yesterday. So now the latest backup of file A is from yesterday, but the latest and only backup of files B and C are from 1/1/2021. When Time Machine goes to perform its next backup, it sees the drive doesn’t have enough space, so it has to make space. Does it delete files A, B, and C from 1/1/2021? Or does it just delete file A from 1/1/2021?
I’m trying to determine whether Time Machine is suited for long-term, archival, or permanent backups. If not, please let me know if there is a method that is better suited for “forever” backups, and how that is done and maintained.
Yes. Time Machine should try to keep one backup of everything as space allows.
So it should have at least one copy of every file you currently have on your drive. Then it should also have at least one copy of files you no longer have on your drive because you have deleted them. But it gets rid of those deleted files first. That's when you start to get warnings.
In addition, it keeps multiple version of files as they change and as space allows.
Time Machine is NOT an archive. It is a backup. Use it to protect yourself from disasters, whether it is hard drive failure, a stolen computer, or just you accidentally deleting a file.
For a permanent archive of files you no longer want on your drive, you should never rely on Time Machine or any "backup" solution. That is not what backups are for. Get an other drive and archive files to that or find a similar online solution.