OS X Yosemite: Choose a backup disk and set encryption options
Turn on Time Machine, add a backup disk or change to a different one, or adjust other settings.
Choose Apple menu > System Preferences, then click Time Machine.
Click Select Backup Disk, Select Disk, or Add or Remove Backup Disk.
The options you see depends on whether you have one or more backup disks already set up.
Choose a disk from the list of backup disks and available disks.
Time Machine can’t back up to an iPod, or a disk formatted for Windows. If you connect a disk formatted for Windows, it can be reformatted (permanently removing all data) to a Mac filesystem format and used as a backup disk.
For increased security, select “Encrypt backups.”
Encryption is available for Time Capsule, disks connected to another Mac on your network, and disks partitioned with the GUID Partition Table (GPT) scheme and connected directly to your Mac.
If the checkbox is dimmed, hold your pointer over it for a moment to see an explanation; for example, the selected disk may need to be reformatted or repartitioned. If no explanation appears, the selected disk doesn’t support encryption.
Click Use Disk.
If you already setup one other disk, click Replace [disk name] or Use Both.
If you already have multiple backup disks, you don’t see this option.
If you’re asked to reformat the disk, either reformat the disk or choose a different disk.Important: Reformatting erases all files on the disk, do it only if you no longer need the files or have copied them to a different disk.
If you selected “Encrypt backups” (in step 4), type a password for the backup disk.
You may need to provide the password when you connect the disk to your Mac, or when you disconnect the disk or restart your Mac.
If you previously used a Time Capsule or networked disk for unencrypted backups and want to turn on encryption now, Time Machine must first erase the unencrypted backup before starting an encrypted backup.
To use multiple backup disks, repeat these steps for each disk.
Time Machine works best if you use your backup disk only for Time Machine backups. If you keep files on your backup disk, Time Machine won’t back up those files, and the space available for Time Machine backups is reduced.