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I'm going to format my iMac today and I've just backed up to an external drive using Time Machine.

But I am using encrypted backup, so Time Machine needs to encrypt the whole drive (after it has backed up). Right now, it says "Encrypting.... 35%". So it will take a long time.

Can I safely format now and still use the Time Machine backup, even though it's not fully encrypted? I mean, will it work the same way as if it was encrypted?

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I'm not completely sure but I wouldn't do that! It will result in a hybrid backup, half encrypted and half not. – Maverik Oct 17 '12 at 10:56
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Ideally, the CoreStorage encryption should continue after you reconnect the Time Machine drive after your format. But practically, it's a risk that you shouldn't take unless you have other backups to recover from (like a SuperDuper! or Carbon Copy Cloner bootable clone).

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