Can I backup my iphone to external HD

Yes. You can but you need to be well versed with the Terminal app. If you mess-up you can never back up again even on your Mac.

You will have to create a symboling link of the backup. See the Step 3 process on --> https://www.imore.com/how-move-your-iphone-or-ipad-backups-external-hard-drive

Alternatively...

For iPhone backup using iTunes (Transfer to external Hard Disk)

1. 1. Open Finder > Go > Go to Folder... > [~/Library/Application Support/MobileSync/]
   2. Open Backup Folder
   3. Copy Paste the BackUp file to any External Hard Disk