use usb drive to back up ipad
can you attach a USB drive or ethernet hub to an ipad or iphone
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An external HDD or SSD can be connected to your iPad with the Apple adapter appropriate for your model of iPad - if the HDD/SSD is both partitioned and formatted appropriately (i.e., a single partition - formatted as FAT32, exFAT (FAT64), APFS or HFS+).
You should also note that the iPad is not capable of providing sufficient power to run an HDD. If the HDD does not have its own source of power - then external power must be supplied to the HDD using a connecting adapter (e.g., an Apple Lightning to USB3 Camera Adapter and Apple Charger of at least 12W) - or by connecting both the iPad and HDD to a powered USB hub.
If you iPad has Lightning Port, you will require a Lightning to USB 3 Camera Adapter:
https://store.apple.com/xc/product/MK0W2ZM/A
If you have recent model iPad or iPad Pro with a USB-C port, you will in all likelihood need to connect both your iPad and HDD/SSD to a powered USB Hub with appropriate USB cables.