If you formatted it as Windows format other improper formats, you have to erase and reformat the external hard drive with Disk Utility. You external hard drive should be formatted as Mac OS Extended (Journaled).If you don't see this message, click Apple menu > System Preferences and select Time Machine, click 'Select Backup Disk' and select the external hard drive for Mac backup on the list, and click Use Disk.Check out our list of best external hard drive for Mac. Your external hard drive should at least be the same size as your Mac's internal drive.You can check the box 'Encrypt Backup Disk' and enter password to encrypt the portable hard drive. Step 1: Connect external hard drive to Mac via USB, FireWire, Thunderbolt or so, and a message will pop up, asking whether you want to use the drive to back up with Time Machine. You have to format your new external hard drive for Mac before backup. Follow Mac backup steps with Time Machine below. It's one of the easiest way to save your Mac data to SSD or HDD, including mac apps, media files, DVD backup, documents and other. Time Machine is a built-in tool on Mac, which works with internal and external disk drives to backup and restore data for macOS. How to Backup MacBook Air/Pro/iMac to External Hard Drive with Time Machine

We also provide tips and fixes for Mac backup problems, Mac not recognizing external hard drive error and more for your reference. In this tutorial, you'll learn how to backup Mac to external hard drive (Western Digital, Seagate, Toshiba etc.) with or without Time Machine, whether they are movies, videos, music, movies on DVDs, iTunes library, iTunes purchases/rentals, Mac apps, documents, or games. If you haven't made any backup for your Mac, you'll regret when your most important data is lost or deleted accidentally.

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