How can I backup my iPad to my external hard drive?

You don't necessarily have to have a backup in place first, but you do need enough free space for the backup that iTunes is planning to make. If it thinks, for example, that the backup will need 10Gb then you'll need at least 10Gb (and whatever safely margin iTunes adds) free on the system drive before it will attempt to commence the backup. Using the symbolic link will mean that the data is actually placed on the external drive. If you don't already have your iTunes library on your external you could potentially move that. Perhaps there is other data that you can move, or you can uninstall software that you don't use. See also the Windows disk cleanup tool.

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See Relocate iOS device backups. Review the rest of the Make a split library portable user tip for advice on organizing the library on external storage.

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