iPad external storage
can an external drive be used with an iPad Pro 12 2021
iPad, iOS 9
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iPadOS provides limited support for external storage.
External storage - such as Flash, SSD and HDD - can be connected to your iPad with the Apple adapter appropriate for your model of iPad. The external storage must be both partitioned and formatted appropriately (i.e., a single partition - formatted as FAT32, exFAT, APFS or HFS+). You should note that external storage devices cannot be formatted by an iOS/iPadOS device.
You should also note that the iPad is not capable of providing sufficient power to run some storage devices that require higher power than the iPad can natively supply. If the storage device does not have its own source of power - then external power must be supplied 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 Storage device 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 2018/2020/2021 model of iPad Pro with a USB-C port, you will in all likelihood need to connect both your iPad and HDD (a high power device) to a powered USB Hub with appropriate USB cables.
Some additional information can be found here:
Connect external devices or servers with Files on iPhone - Apple Support
https://www.imore.com/how-use-external-hard-drives-your-iphone-or-ipad