External drive support for iPads/iPadOS is still in its infancy.
If this a conventional mechanical spinning platter hard drive, iPads may not have a powerful enough internal power supply to supply enough power to the drive to get it spinning.
You may need to plug that drive into its own separate power source using the hard drive's own power adapter or using an externally powered USB-C hub.
NO iPad, even the more expensive iPad Pro, is a complete a full replacement for a MacBook/laptop computer.
In addition the external drive needs to be formatted into one of these file/data formats
Apple native macOS format
APFS
FAT 32
ex fat
Apple Website external drive help link here.
Connect external devices or servers with Files on iPad - Apple Support
Best of Luck to You!