Can I back up my macbook offline
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howapple - hello
Not if it won't turn on. If it has a removable drive you can take out the drive, insert it into an external enclosure, connected the enclosure to another computer, then copy the data to the other computer. If the issue with your computer is a failed hard drive you won't be able to retrieve the data.
The time to back up your data is before you experience a computer problem not after the fact.