The Unofficial Apple Weblog (TUAW)

Q: What is the suggested size of an external drive to be used with TM? Does it have to be at least the same size of the startup drive? Twice as big? Half as big?

A: Apple hasn't published a recommended ratio of source to target size for Time Machine, but "as big as you can afford" is a good place to start. With TM's backup approach, any files changed in the last hour are backed up -- this lets you 'scroll into the past' and see the state of your files at an earlier time. With the default hourly snapshot scheme, frequent modifications to large files will eat up disk space in a hurry. Some especially bulky and dynamic files (I'm looking at you, Entourage's main database) will probably have to be excluded from TM backups, reducing their utility somewhat.

I'd recommend a baseline of 2x your source drive for your backup drive; depending on your level of activity, that may last you for a while. TM tries to keep hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for everything older than a month -- it will warn you before it begins deleting older backups -- so a twice-as-big external should do pretty well. Apple's got a deal on a 500 GB external Iomega drive for $140 right now (thanks Evan); you can use USB or Firewire drives for TM, and even back up to another machine or to a shared/SAN volume if you've got the setup.

Q: Can Time Machine "store" the changes even if I am on the road with my Mac and then sync them once I connect the backup drive?

A: Not positive, but I believe the answer is yes, after a fashion. You won't get the same hourly snapshots that would kick in if the backup drive was connected, but TM will still note all the changed files. Once you get back and plug in, you'd get a 'catch-up' backup with all the changes between disconnect and reconnect. This means that you can't roll back to intermediate file states that happened on the road, but it's better than nothing.

Q: Does the HD have to be formatted in a particular format or can the software work with "any" HD?

A: Apple says HFS+, and that's all that's required. USB, Firewire, remote volumes, whatever you like; the TM backup is just a big folder as far as the drive is concerned.

If you've got more Time Machine questions, let us know!

Update: Some additional questions from the comments..

Stack -- How will Time Machine backups on a server work? For instance, I'll be running Time Machine with two drives on my Leopard server. The second drive will be a Time Machine drive for the main drive. Can I also share that same Time Machine drive on the network for my laptops to back up to?

I've heard that TM will prompt you to backup your main drive prior to installing Leopard so that you can revert back to tiger if you run into issues with the install. Is this true? And if you backup with TM, then do a clean install of Leopard can you migrate you old apps over?

You are prompted for TM setup during your Leopard install. I'm not sure how well a "rollback to Tiger" via TM would work. As noted above, you can use TM backups as a source for Migration Assistant, so you could migrate your applications that way after a clean Leopard install.

I only hear about USB or firewire disks - does anybody know whether TM will work with a harddrive attached to my Airport extreme base station?

what about backing up to an hdd connected to an airport extreme? i think i remember steve saying this was supported way back at wwdc when he announced leopard.

My wife and I both have powerbooks. I'd like to set up a new airport extreme with a big external drive. 2 questions.

1. Can Time machine work over airport?
2. If it is running allthe time, what happens if I sleep, or shut down while TM is saving?

Yes (yes yes). An HFS+ drive attached to an Airport Extreme is a valid target for TM backups, and should work just fine. The jury is still out on drives attached to the Airport Extreme -- there have been conflicting statements. Current spec does not include the AEBS, but it may support it later. Regarding sleep or shutdown, TM is supposed to be smart enough to pick up where it left off if a backup pass is incomplete. Since TM gets its information about changed and updated files via the same APIs that Spotlight uses to index your drives, it should keep track of what's new without too much trouble, and start at the right spot.

I can't verify this one way or the other right now. I think you can select additional sources for backup, but I'm not sure. If not, there are many other excellent choices for backup apps. Verified, TM defaults to backing up every mounted local drive on your machine (except the backup target); you have to exclude the drives you DON'T want to back up.

The question that seems to stay unanswered is how will TM work with network drives? We're looking for a LaCie Ethernet RAID for backups for the office - will TM work with that?

The question is answered -- any HFS+ volume, local or remote, will work for Time Machine. However, the LaCie Ethernet RAID does not support HFS+ or AFP directly (see specs here) and so I would NOT expect that particular device to work. Apple says: "You can designate just about any HFS+ formatted FireWire or USB drive connected to a Mac as a Time Machine backup drive. Time Machine can also back up to another Mac running Leopard with Personal File Sharing, Leopard Server, or Xsan storage devices."