to commonly asked questions.

Time Capsule HD erase walkthrough

How do I erase my Time Capsule HD?

Open Airport Utility then select your Time Capsule router and hit "Edit" on the popup window.

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Next, select Disk from the menu items and then select the drive listed. Uncheck "Enable File Sharing" if you want to remove the ability for others to access it from the network. You can also select to secure the disk with its own password instead of "device password". This gives you the ability to further secure the HD with a different password other than the wifi password. Then hit "Erase Disk".
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Now, rename the HD to anything you'd like and select your Security method. Quick Erase will delete the file map but the data still exists on the drive. This is fine for 99% of users. If there is extremely sensitive data on the drive, then you may choose to zero out the drive with one or more passes. This takes several hours to complete however.
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Video tutorial here:

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