iPad Pro and External disks
How can I access files on an external drive from my iPad Pro?
iPad Pro 12.9-inch, 3rd Gen, Wi-Fi
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That would largely depend upon your model of iPad.
Connect external storage devices to iPad - Apple Support
https://www.imore.com/how-use-external-hard-drives-your-iphone-or-ipad
iPadOS provides limited support for external Hard Disk storage.
An external HDD can be connected to your iPad with the Apple adapter appropriate for your model of iPad - if the HDD is both partitioned and formatted appropriately (i.e., a single partition - formatted as FAT32, exFAT, 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/uk/xc/product/MK0W2ZM/A
If you have a 2020 iPad Air4, or a 2018/2020 model of iPad Pro with a USB-C port, you will in all likelihood need to connect both your iPad and HDD to a powered USB Hub with appropriate USB cables.