Hard drive on iPad Pro

There’s your problem. Whilst NTFS is supported by your PC, NTFS is not supported by iPadOS.

As stated in my initial reply, your external storage device(s) must have only single partition and be formatted as FAT32, exFAT or APFS (APFS is Apple proprietary).

The only universal formats that work cross-platform are FAT32 and exFAT; both PC and Mac can format devices to either of these formats.

That adapter will not work and the Windows NTFS format is not supported on Apple mobile device.

Also, unfortunately, initial formatting/re-formatting of any externally connected drives to be used on Apple Mobile devices MUST be formatted using an Apple Mac computer.

No option.

You need this adapter.

https://www.apple.com/shop/product/MK0W2AM/A/lightning-to-usb-3-camera-adapter

And tbe drive needs to be formatted in either

Apple native macOS format

Apple APFS

Fat 32

Ex fat

You need to find a family member OR friend that owns a Mac to help you format/re-format these drives

OR

Go online, locate your nearest, “local” Apple Store location and call and make an appointment to go into the store and have Apple Genius bar employees help you with formatting/re-formatting these drives for use with your iPad Pro.

Best of Luck to You!

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 or APFS). 

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 the 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.

I hope you find this to be helpful in providing some useful clarity.

Are you using a genuine Apple Lightning to USB3 Camera Adapter that incorporates both a USB Type-A and pass-through Lightning power port - or something else?

The earlier version Lightning to USB Camera Adapter, with only a USB connection, is inadequate - unless you are also using a powered USB hub, to which everything must be connected.

Other than incorrect format or partitioning, which will inhibit use of any external storage (HDD, SSD or Flash), the issue you describe is typical of inadequate power provision.

Subject to the limits imposed by your subscribed iCloud Storage tier (5GB is free - paid tiers being 50GB, 200GB and 2TB), yes, you can copy data from your PC to iCloud storage.

Once data has been uploaded to iCloud, the data/files will be accessible by your Apple devices.