Information Technology Reference
In-Depth Information
Note: Before you detach a drive's cable from your Mac, eject the
volume by clicking the eject
icon next to its name in the Devices
section of the Finder sidebar. If you fail to do this, you could interrupt
Mac OS X in the process of writing data, potentially damaging your
Time Machine backups.
However, Time Machine has an idiosyncrasy that may cause some
unexpected behavior if you move a given drive between local and
network connections.
As I mentioned in Restore Files without Time Machine , Time Machine
stores your backups in a folder if you've selected a locally connected
drive, but in a disk image if you're backing up over a network. Now
let's say you want to save time on your initial backup by seeding it
onto an external drive and then moving that drive to another computer
where you'll share it over the network. When you connect a drive
locally and let Time Machine back it up, you get a folder for that
machine inside the Backups.backupdb folder. So far so good.
Then you move the drive over to another Mac, and share it over the
network. On the first Mac, you tell Time Machine to use that shared
disk. It will do so, happily, but it won't recognize your existing backup;
instead, it'll start a new one, from scratch, in a freshly created disk
image! The reason for this behavior is simple: Time Machine has no
way to know that the computer you're backing up over the network is
the same one you were backing up locally. (But don't worry, I provide
a solution to “beat the system” just ahead.)
Now let's reverse the situation. You back up over the network first ,
and then later plug the drive directly into your Mac and select it as a
Time Machine destination. In this case, Time Machine does recognize
that you're backing up the same Mac, and does pick up with the
existing backup where you left off, because it sees the MAC address
of the computer's Ethernet card (regardless of which network interface
you're using at the moment) in the disk image on the backup disk.
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