As a simple and effective way to backup a drive, Time Machine has been a welcome new feature in Leopard. However, it does not always work smoothly, especially if your backup drive is accessed wirelessly via an AirPort network.
In my own experience, I have used Time Machine with a hard drive connected via the AirPort Disk feature of an AirPort Extreme Base Station. After some initial success, Time Machine began sticking at the “Preparing…” stage of starting a backup. This lasted several hours with no sign of progress. Eventually, I gave up.
I decided to reformat the drive and start over with a new backup. The symptom soon returned. On the most recent occasion, when I attempted to reconnect to the drive to Time Machine, I got a kernel panic.
What does Apple have to say about all of this?
First, as reported at TidBits, Apple asserts that using Time Machine with a drive connected to an AirPort Extreme Base Station is “unsupported.” Apple only supports AirPort network backups using Time Capsule. Actually, this was Apple’s position as far back as October 2007, when Leopard was first released. At that time, you couldn’t even get Time Machine to recognize a drive connected by way of AirPort Extreme.
Using the latest Mac OS X software and firmware, however, Time Machine does recognize and connect to such drives. Yet Apple continues to contend this setup is unsupported. I’m not exactly sure how to interpret this, other than to expect little help from Apple here (that’s what “unsupported” means after all).
Regardless, Apple acknowledges that a prolonged “Preparing…” stage can occur even with a supported Time Machine setup, explaining that it can happen if “Time Machine loses track of files and folders that have changed since the last backup.” The long wait is the result of Time Machine refiguring what needs to be backed up.
How might this forgetfulness occur? It can happen if you do something improper, such as removing a disk before selecting to eject it. More worrisome, Apple says it could happen simply as the result of going several days without using Time Machine! So, if you intend to use Time Machine, be prepared to have it running all the time. This may have been behind some of my problems, as I was periodically turning Time Machine off and back on.
If you do get stuck at “Preparing…,” Apple’s advice is to wait it out, no matter how long it takes. Why? Because if you interrupt the backup, the “Preparing…” hang-up is likely to last even longer the next time. Not necessarily so, say Time Machine users. Reports at Apple’s Discussion Boards and on sites such as MacFixIt claim that simply turning a backup drive off and back on was sufficient to push Time Machine past the “Preparing…” hump on the next backup attempt.
If nothing seems to work, you can always start over by using Disk Utility to reformat the drive. If you do, just make sure that you do not format the drive using a Master Boot Record partition. Use GUID (for Intel Macs) or Apple Partition Map (for PowerPC Macs).
Finally, what about my kernel panic? This has also been reported with Time Capsule, so it is not simply a result of my using an “unsupported” setup. The cause appears to be corrupt data on the drive. Various potential solutions have been suggested. None appear guaranteed to work. Again, starting over with a newly formatted drive is probably best at this point. If the problem persists after reformatting, as it has for some users, you’ll probably have to wait for a bug fix from Apple before it gets completely resolved.