Restoring iPhone Backup from External Hard Drive
To restore iPhone Backup from a Hard Drive you can just copy and paste the backup folder from the Hard Drive to the iTunes Backup folder on your Computer.

On both Windows and macOS, iOS backups are stored in a MobileSync folder.
On macOS, iTunes will store backups in /Users/[USERNAME]/Library/Application Support/MobileSync/Backup. (macOS 10.15 creates backups using Finder rather than iTunes, but these backups are stored in the same place.)
Under Windows Vista, Windows 7, 8 and Windows 10 iTunes will store backups in \Users\[USERNAME]\AppData\Roaming\Apple Computer\MobileSync\Backup.
The Microsoft Store version of iTunes stores its backups in \Users\[USERNAME]\Apple\MobileSync\Backup.
Under Windows XP, iTunes will store backups in \Documents and Settings\[USERNAME]\Application Data\Apple Computer\MobileSync\Backup&quot;.

https://reincubate.com/support/how-to/where-does-itunes-store-backups/
In Windows you can just open Run and type %AppData%\Apple Computer\MobileSync\Backup (Won't work if installed from Windows 10 Store)
Or else you can do the easier way that is Hold Shift when pressing the Restore button in the Summary section of the iPhone