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I have a 500 GB WD My Passport with two partitions on it, one of which is set as a Time Machine backup drive. The problem is that the backup doesn't start automatically when I connect the HDD. Any idea why?
If it helps, the only thing I did was to partition it, I haven't formatted it, so the SmartWare drive kicks in every time I connect it.

Thanks!

Environment: Last generation MacBook Pro; Mac OS X 10.6.8.

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If you look in /private/var/log/system.log for items with the string backupd - you will see more detail on the steps Time Machine is taking or perhaps some error messages. – bmike♦ Jun 27 '11 at 18:07

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(Edit: Lion makes this less of an issue with local snapshots possible as well as the ability to script the starting with tmutil startbackup - the rest of this answer is in regards to Snow Leopard implementation of TimeMachine)

Time Machine sets a timer based on when the mac is booted, so it's very unlikely the system looks to start a backup when the device is attached.

Instead, it is designed to commence a backup periodically and will log messages and skip a backup if there is a problem or the drive isn't attached. Once one or more backups gets skipped, perhaps it might start one sooner when the volume it expects is attached. I just checked on one mac that backs up to a local disk. When I log out all users, it doesn't see the disk at attached, so it skips each back up. When I log back in, that user log in mounts the disk and Time Machine started a backup immediately rather than waiting the "rest of the hour" for the backup.

This is how it is supposed to work.

I would initiate a backup using the control panel or the menu item to be sure there isn't an error condition - but once you rule that out, it's just a matter of time until the hour passes and your backup will start.

Have you made sure there isn't some other issue by checking in the console app? Also, do you let this mac sleep, or will it stay running for several hours and still skip a backup when the drive is connected?

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I had an older HDD that was setup as a Time Machine backup drive and the backups used to start immediately after it was connected/detected. – Alex Jun 27 '11 at 18:25
I believe there is (and certainly was at one point) a mechanism to kick start things when the drive is gone for some days time, but can't pin it down in code or using google at the moment. Also, how the drive is connected and whether a user is logged in also seems to matter. – bmike♦ Jun 27 '11 at 19:36
Connected, as in the type of interface used (e.g. USB/FireWire)? – Alex Jun 27 '11 at 20:53
TimeCapsule or Connected to OS X Server as a network share and using sparse disk bundle versus a physical disk over USB/FireWire/ThunderBolt/whatever where the backup is stored as a folder. – bmike♦ Jun 27 '11 at 20:56
I agree with @bmike's answer. I would only add that in addition to a variable start up time, once it decides it needs to start, then it also spends some time preparing the backup (i.e., figuring out what has changed). If you haven't backed up recently, this step alone could take several minutes. A final note to the original person who was asking the question, you can kickstart the Time Machine backup process by selecting the Time Machine menu (from your menu bar) and select "Back Up Now". You can also kickstart a backup from the Time Machine Preference pane. Hope this helps. – Christian Correa Jul 28 '11 at 4:31

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