ITUNES SAYS DISK IS FULL AND CANT COMPLETE RESTORE

The first stage of restoring an iOS backup involves making an archive copy of it. If your backup is 20Gb, you need at least another 20Gb of free space before you start. See Relocate iOS device backups and move the backup data to a drive with more room. Note iTunes still checks for free space on the system drive.

Review the rest of the Make a split library portable user tip for advice on organizing the library on external/secondary storage.

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