Information Technology Reference
The Time Machine Schedule Problem
Time Machine says it saves hourly backups for 24 hours, daily
backups for a month, and weekly backups until your disk is full.
On the surface, that seems reasonable, but if you look at the
details, there's a catch.
Time Machine makes a new backup every hour that your Mac is
on and awake. With each run, Time Machine also deletes the hourly
backup from 25 hours ago, unless it was the first backup of that
particular day. Thus you always have hourly backups for the last
24 hours, as well as a single hourly backup (i.e., from just the first
hour of the day) for each of the past 30 days. After a month, Time
Machine deletes the oldest of the daily backups, but it preserves
the first daily backup from each week as long as there's disk space
Now picture this: at 8:30 PM on Monday you create an important file.
When Time Machine runs next (at, say, 9:00 PM) it backs up that
file; so far, so good. Now, at 9:30 PM, you delete the file, either
intentionally or otherwise. No problem: it's still in your backup. Of
course, none of the hourly backups for the next 24 hours includes
your file, because it had already been deleted, so the only copy Time
Machine has is in that initial hourly backup. At 10:00 PM on Tuesday,
Time Machine erases that backup from 25 hours ago—the only one,
from 9:00 on Monday, that contained your important file. Because
that file wasn't in the first hourly backup of any day, it won't be there
tomorrow if you suddenly realize you need it, even though Time
Machine backed it up yesterday!
So there are ways that files can fall through the cracks. Time Machine
backs them up, sure, but then, because of the way it deletes old
backups, it may remove your essential file from the backup before
you need it. (And that's true, by the way, when Time Machine deletes
old daily and weekly backups too.) Plus, if a file exists for less than
an hour, and therefore isn't around for a single backup, Time Machine
won't help at all.
The lesson? First, supplementing your Time Machine backups is a
good idea. And second, get in the habit of hanging on to files for at
least 24 hours before you delete them!