PORTABLE DRIVE FORMAT
Do you have to format portable hard drive in order for iPad pro to see it? What format to use?
Can you use USB for data transfer instead of just adding-on peripherals?
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For iPadOS13.x to access external media, it must be formatted as FAT, FAT32 or exFAT (FAT64) for universal readability across Apple, Microsoft or Linux systems - or additionally APFS+ if only used with Apple systems. NTFS and other formats are not supported. Drive formatting must be performed on other computing devices; drive formatting actions are not supported by iPadOS.
Additionally, for compatibility with iPadOS, the drive must only have only single partition.
HDDs tend to more problematic with iPadOS/iOS13.x than flash-based storage media - such as USB thumb-drives. You will also likely need to provide additional power to the HDD via a powered USB hub - as the iPad will not be able to provide sufficient power.
Dependent upon your model of iPad, you will need to use an appropriate adapter (such as the Apple Lightning to USB3 Camera Adaptor - for devices that use a Lightning connector) or directly using USB-C for the most recent models of iPad Pro. When using USB-C, you may still need to use a self-powered HDD or powered USB hub to provide adequate power to the system.
I hope you find this helpful.