Back up multiple internal drives on your Mac with Time Machine
Users with two internal drives in their Mac may want to use Time Machine to back up each drive. The default settings of Time Machine do not make this option clear, but it is indeed possible. This article will show you how.
Apple Support Discussions user "ProfWhiplash" asks:
"I would like to back-up my two internal HDs using TM, but the preference do not allow me to select the other drive. My internal drives are 230 GB and 500 GB respectively. My TM drive is 1 TB (plenty of room). How do I designate TM to back-up both drives?"Solution
By accessing your Time Machine preferences you can add a second drive to Time Machine's backup process. Apple Support Discussions user "Pondini" suggests:
"When you go to TM Preferences > Options, click the plus sign at the lower right, then your computer's name in the sidebar, does the 2nd internal appear at all?Be sure when you are using Time Machine that your backup drive contains enough space to fit both of your internal drive backups (and is formatted correctly as described above). Allow plenty of time for Time Machine to create the initial backups (for two drives expect an overnight process).Or is it shown, but grayed-out?
If it doesn't appear at all, does it appear in the sidebar of Disk Utility (in your Applications/Utilities folder)?
If it does appear in Disk Utility, select the top line, with the make and size. Towards the bottom, the Partition Map Scheme should be GUID for an Intel Mac, or Apple Partition Map for a PPC Mac.
Select the line for the partition (indented, with the name). Towards the bottom, the Format should be Mac OS Exended (Journaled)."
Resources
Read this Apple Support Discussions thread about backing up multiple onboard drives with Time Machine.
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Resources