When I backup my iPhone 6s to my iMac, I get an error message saying that some files cannot be copied to the MacHD because the files can't be found (I assume on the HD). Then after i check the "ok" box it then gets stuck in step #6 of the backup. I then have to cancel the backup and eject the phone.
iPhone 6s, iOS 11.4.1
If the above doesn’t help I would try deleting your existing BackUp/BackUps because there may be some corruption (couldn’t see that advice in above link).
Locate backups of your iPhone, iPad, and iPod touch - Apple Support
You can delete backups from within iTunes or navigating direct via Finder (the above link should help). Once deleted I would restart both your iPhone and iMac before attempting the backup process just to make sure nothing is retained within memory.
iPhone 6S Never Completes backup
