Unable to backup my iPhone using iTunes. Error: Insufficient space on my Mac

I am trying to backup my old iPhone 7 to my Mac, so that I can restore it to the new iPhone XS. iTunes keeps giving me an error stating Unable to backup due to insufficient space on the computer. The iPhone that I am trying to back up is 128 GB capacity (with 70 GB free, and I have 128 GB free on my Mac.

What's the probable problem here? Software on all of my devices are up-to-date (macOS Mojave on my Mac and iOS 12.1 on the iPhones).

Solution 1:

According to your information provided

You have 17 GB free space on the Disk.

You also have 128 GB of which 111 GB can be purged (freed).

To purge go to About this Mac > Storage > Manage

Decide what to do by following the instructions.