For iPadOS 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.
Some external drives use write-caching. If not properly dismounted prior to disconnection, the drive may be corrupted - and not be recognised by the iPad upon reconnection. Unlike Windows PC and Mac, iPadOS does not have a Safe Remove function; for drives that employ write-caching, the only method of ensuring a safe dismount is to shut-down the iPad before disconnecting the storage device from the iPad.
If the drive formatting has been corrupted, the drive formatting must either be “repaired” by a Mac or Windows PC - or completely reformatted.