Transfer photos to stroage disc

What, specifically, are you attempting to achieve?

Are you looking to archive your locally stored photo's - or move them to some form of external storage (such as a portable flash drive) so as to free-up local storage space on your iPad? While it is entirely possible to do so, there are some consequences - such as the photos no longer being visible or directly accessible from the iPad's Photo App. Only photos stored locally on your iPad can be seen in your Photo App's photo library.

The mechanics of connecting USB storage devices to your iPad are straightforward - but, depending upon your iPad model, may require use of a suitable Adapter to connect the USB storage device to the iPad.

The tags appended to your post suggest that your iPad has iPadOS 16 installed. If you have an older model iPad that has a Lightning connection, you will need to use an Apple Lightning to USB3 Camera Adapter to connect USB devices to the iPad. This adapter has both a USB 3.0 Type-A connector and an additional Lightning port to supply power, from your Power Adapter, to both the iPad and connected device. The Power Adapter must be rated at 12W or more.

• Lightning to USB 3 Camera Adapter

https://store.apple.com/xc/product/MK0W2ZM/A

If instead you have one of the newer iPad models with a USB-C connector, you can connect USB storage directly or using a simple USB Adapter for the older USB Type-A connections used by many storage devices. In some cases you may need to utilise a powered USB hub (i.e., it has its own source of power) - connecting both the iPad and the storage device to ports on the hub.

With the benefit of more information as to precisely what you wish to connect to your iPad, specific connection advice can be offered. Either way, please ensure that your iPad has been updated to the most recent version of iPadOS that it can support. For information, here is a list of the highest version of iOS supported by the older models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated:

• iPad Air1*, iPad mini2* and iPad mini3* - iOS 12.5.7
• iPad Air2*, iPad mini4 - iPadOS 15.8.5
• iPad5*, iPad Pro (9.7"/12.9" 1st generation)* - iPadOS 16.7.12
• iPad6, iPad Pro 12.9" 2nd generation*, iPad Pro 10.5" - iPadOS 17.7.10
• iPad7 - iPadOS 18.7.3

(*) obsolete

Newer models are all able to be updated to the current version - at the time of writing, this being iPadOS 26.2:

• iPad Pro 13" (M4/M5)
• iPad Pro 12.9" (3rd generation and later)
• iPad Pro 11" (1st generation and later + M4/M5)
• iPad Air (3rd generation and later + M2/M3)
• iPad (8th generation and later + A16)
• iPad mini (5th generation and later + A17 Pro)

If your iPad is capable of update to iPadOS 17 or later, exporting photos to external USB or cloud storage is relatively easy - as documented within the iPad User Guide:

Import and export photos and videos on iPad - Apple Support

That said, there are many third-party File Manager Apps that can backup or move data to a variety of external storage options - including third-party Cloud services, external media devices and network storage. Using a third-party file manager also resolves a potential difficulty outlined earlier, namely the inability of the native Photos App to access or view photos that are not locally stored on the iPad; the File Manager App (such as Stratospherix) incorporates advanced photo and file viewing capabilities directly from the App.

Of those available from the App Store, 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.

If you have a computer, you can transfer the photos to your computer, and back them up from there.

Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support

E.g., if you had an iPad and a Mac, and both were set up to use iCloud Photos (for the same Apple Account), iCloud Photos would try to maintain the same set of photos on both. You might want to set the Mac's Photo Preferences / Settings to "Download Originals to this Mac" so that the Mac had full-size copies. This is what I do for transferring photos from my iPhone to my Mac, or vice versa.

In iPadOS 17 and later, you can export photos directly from the iPad to an attached storage device.

Import and export photos and videos on iPad - Apple Support

According to your tag line, you are running iPadOS 16, so this option might not be available to you. I think there may be some third-party products that consist of a USB flash drive plus a special app (downloaded from the App Store) to let you transfer photos even on versions of iPadOS older than iPadOS 17. But I haven't kept track of them.