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Whenever a backup is run, Time Machine says "Preparing Backup...". During this stage, it looks for what files have been changed since the last backup by comparing changes in the Event Log (I assume) with its cache of files on the system.

Because of the cache, the Preparing stage is usually quite fast (30 seconds), but for some reason, sometimes when I restart from Boot Camp (Windows 7) into OS X, Time Machine's cache is wiped clean or corrupted. I know this because the Preparing stage takes 10 minutes after booting back into OS X.

So, that's why I'm wondering where the cache is located so perhaps I could determine why it's sometimes corrupted.

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Are you mounting the Mac partition while you are using Bootcamp? If you are and the drive is not set to read only then time machine will do a full scan upon reboot. – l'L'l Jul 13 '12 at 13:52
Interesting. I do have MacDrive installed, which essentially lets me read and write to HFS+ drives while in Windows 7/Boot Camp. Could that be doing it? Although, as I said, this doesn't always happen, yet MacDrive always starts up automatically in Windows, so perhaps it's not it after all? – Gary Jul 15 '12 at 3:45
up vote 1 down vote accepted

There could be any number of different factors involved in why Time Machine is doing a full scan after using Windows/Bootcamp. From what you describe it seems out of sync, as if your system is telling Time Machine 'this volume is not like it was, so do a full scan'. One of the first things that might give you an idea of what is going on would be to open Console immediately after Time Machine kicks in with 'Preparing...'. By observing fseventsd, system.log, and backupd you should be able to get a better picture of what might be taking place. You can type any of those terms in the search box of Console to see a consolidated list of those events.

This article is also an excellent in-depth guide to Time Machine; it covers just about everything you could imagine, so take a look.

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Thanks, I'll try these out. The one particularly useful tip was to look at the Console; I opened, clicked on system.log, and searched for fseventsd, which provided me with some useful messages that I'll look into further. – Gary Jul 16 '12 at 17:41
This is pretty much correct, but I'd like to clarify that what's happening isn't a Time Machine cache getting corrupted (as you state in your question), but the filesystem event log being out of sync. OS X uses fseventsd to log everything that happens to the filesystem, when OS X is running. If you're in Windows and the disk is mounted read/write, Windows will be able to modify the files, but will not update the event log! As such, Time Machine must re-scan the drive next time it tries to back up, to see what changes might have occurred. – cablesm Nov 29 '12 at 18:31

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