How can I backup my ipad to an external hardrive?

This support page outlines the natively supported backup methods for iPad and iPhone:

How to back up your iPhone, iPad, and iPod touch - Apple Support

Native iOS/iPadOS backup and restore processes are dependent upon iCloud, or connection to either Windows PC (with iTunes for Windows installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS).

Natively supported methods offer full system backup - consisting of data and device settings.

How to back up your iPhone, iPad, and iPod touch - Apple Support

It is possible to use a third-party App to backup data (but not device settings) to a variety of external storage options - including third-party Cloud services, external media devices and network storage. One App that comes readily to mind is FileBrowser Professional by Stratospherix:

https://apps.apple.com/gb/app/filebrowser-professional/id854618029

You would be well advised to review the extensive online Manual and help files to determine suitability for your needs.

https://www.stratospherix.com/products/filebrowserprofessional/filebrowser-professional-user-guide.php

Speaking of personal experience, I am yet to encounter a file storage or file/photo backup scenario that the FileBrowser App won’t handle. I routinely use USB and third-party Cloud storage for this purpose. FileBrowser also integrates directly with the native Files App, further simplifying file/photo management. There is a learning curve, but it works.

you can buy some icloud storage and backup to icloud

How to back up your iPhone, iPad, and iPod touch - Apple Support

you can't backup ios devices directly to external storage