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I use Mac OS X 10.6.8. I have a brand new external portable hard disk, a Seagate 1 TB, formatted as NTFS. I want to use it as backup drive for Time Machine.

When I open Time Machine, it does not show my hard disk as an option that can be used for backup. Do I have to format my HD to some other type like FAT or EXTENDED? Do I need to download some software for making my HD work as backup disk?

Once it becomes my backup disk, can I use it for storing my other data from Windows or Mac too?

I read about using HFS+ formatted drives, but my Disk Utility does not show that option.

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In order to use your external drive for Time Machine, it has to be formatted with HFS+ (also called Mac OS Extended). OS X won't be able to write to NTFS partitions, and both NTFS and FAT don't support the features Time Machine needs to work properly.

Your only option is to format the drive. Go to Disk Utility, select the drive on the left, and then go to the Erase tab. Select Mac OS Extended (Journaled), which is the default, give it a name, and then click Erase…. Note that this will erase all data on the drive.

You can now use the disk for Time Machine. OS X should prompt you whether you want to set it up as a backup disk.

Windows, however, won't be able to write to this volume anymore. Linux may work, but the drivers for journaled HFS seem unstable, so it's not recommended to use this partition under other operating systems (so as not to screw up your Time Machine backup).

What you can do is partition the drive (see Partition tab) into, let's say two partitions, one being HFS+ for your Time Machine backup, the other being ExtFAT or FAT-32, both of which can be read and written to by OS X, Windows and Linux.

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