Back up was corrupt or not compatible with the iphone?

So a couple days ago I realised that my iphone backups were taking a lot of space up on my macbook, so I did some research and found out i could move the backup folder to my external hard drive. So i followed the steps and moved it over. But now when I go to sync my phone it says that the backup was corrupt or was not compatible with the iphone. I don't know why it's saying that, i have my hard drive plugged in and it should be backing up to the hard drive. I think I might have done a step wrong to move the folder to my hard drive because I have not seen anyone else with my problem.

Not finding the file on the external hard disk. You have to find the destination setting on the iphone backup utility or put the backup file onto the laptop hard drive again, then do the backup. Then put the new backup onto the external drive.
Another cause might be the date that was given as newer in the property of the copied backup file.