iPad and iPadOS provides limited support for external Hard Disk storage.
As the iPad has limited power capabilities, dependent upon the Storage Device, an external power source is often required. For HDD, connected via USB, an external power source is almost certainly required. SSD bulk storage generally offers higher performance, is more robust - and requires considerably less power.
If you have one of the newest iPad Pro models, with a USB-C connector, use of an HDD may necessitate the use of a powered USB hub (i.e., it has its own source of power) - connecting both the iPad Pro and the storage device to ports on the hub.
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
To be recognised by iPadOS, the attached USB storage device must be formatted to FAT, FAT32, exFAT, APFS or HFS+ with a single partition. Microsoft NTFS is not supported. Formatting and partitioning, if required, must be performed using a separate computer; iPad does not provide any facilities to format an external storage device.
Having an iPad with a USB-C port provides useful flexibility in being able to connect to various accessories and devices.
Apple as a manufacturer offer few USB-C hub options, presumably due to relatively wide choice from third-party sources. The better USB Hubs that have a USB-C connection for iPad plus a USB-C PD (Power Delivery) pass-through power connector to which you can connect your USB-C Power Adapter - along with the hub itself often incorporating a combination of additional USB-C ports, USB 2.0/3.1 Type-A ports, Ethernet LAN, HDMI, VGA, SD and microSD card-readers.
Perhaps consider Kingston, Anker, Hyperdrive and Satechi - although there are many others from which to choose. A Google Search for USB-C Hubs compatible with iPad will find a number of articles and recommendations.
Speaking of personal experience, the Kingston Nucleum has proven itself to be particularly flexible for connection of high power USB storage devices - featuring twin USB3.1 Type-A, downstream USB-C and SD/microSD card slots, HDMI, plus a power pass-through USB-C port that can accept +45W from a suitable USB power source.
If looking for an HDMI Adapter, be aware that very few USB-C HDMI adapters/hubs support HDCP 2.0 - absence of which will inhibit the ability to output copy-protected HD media content from iPad to a connected monitor/TV over an HDMI connection.
The Apple USB-C Digital AV Multiport Adapter does support HDCP 2.0, but perhaps lacks some of the additional the flexibility afforded by some third-party USB-C hubs:
https://store.apple.com/uk/xc/product/MUF82ZM/A
Also note that some iPad models that feature USB-C also support DisplayPort Alternate Mode over USB-C. This protocol allows direct connection, with an appropriate Thunderbolt cable, to a monitor that supports DisplayPort - removing the need for HDMI.
