Back Up iPhone to SD card rather than MacBook Air Hard Drive.
Can I back up my iPhone to an SD card instead of my MacBook Air's hard drive?
MacBook Air
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Apple does not recommend that.
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To avoid ruining any of your backup files, here's what you should know:
- The Finder and iTunes save backups to a Backup folder. The location of the Backup folder varies by operating system. Though you can copy the Backup folder, you should never move it to a different folder, external drive, or network drive.
- Don't edit, relocate, rename, or extract content from your backup files. For example, to restore from a relocated backup file, you need to move the backup file back to where it was in the Backup folder or the file won't work. Backups serve as a safe copy of almost all of your device's data and settings. So while you might be able to view or access a backup file's contents, the contents usually aren't stored in a format that you can read.
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copied from: Locate backups of your iPhone, iPad, and iPod touch - Apple Support