The handphone storage is getting bigger and bigger. Latest generation iPhones have storage at 128GB.
To back up the iPhone, a similar amount of disk free space in the My Documents folder. This is usually located in the system drive, C. It is highly unlikely this amount of disk free space is available.
There are numerous websites with step by step procedure to redirect the backup to another drive or partition. I put the link below. I tried the guides and they don’t work with USB drives. The backup job backs up about 2GB and then stops.
The reason was the 128 GB USB drive was formatted in FAT32. For some reason, iTunes only supports 2GB. Re-formatted to NTFS and the job completes successfully.
I would advise keeping the original Backup folder and delete the symlink. So that you can remove the USB drive without warnings.