Reasons why your external hard drive is read-only on iPad:
Incompatible file system
iPadOS provides complete read and write compatibility for external hard drives formatted in Apple APFS, HFS+, Microsoft FAT, FAT32, or ExFAT (64-bit). Starting from iPadOS 15 and later versions, NTFS is partially supported. When you connect an NTFS-formatted flash drive to an iPad, it will be in read-only mode, preventing you from adding files to it.
The external hard drive requires too much power
If your external hard drive is a conventional, mechanical hard drive with spinning platters, it demands more power than what an iPad power supply can provide adequately. This also happens to USB sticks and other accessories, and you may be greeted with a message saying "This accessory requires too much power". As a result, your external hard drive can't work properly when connected to your iPad, appearing in read-only mode.
How to solve external hard drive read-only on iPad
Now you have learned what might cause the issue, it is easy to troubleshoot it.
Solution 1. Format your external hard drive
This solution is useful for an external hard drive using a flash memory SSD. With an iPadOS-supported file system, you will be able to read from or write to your external hard drive.
Step 1. Connect your external hard disk to your PC.
Step 2. Back up all important files on your device since formatting will delete all files.
Step 3. Open Disk Management and find your external hard drive.
Step 4. Right-click on the external hard drive and choose Format.
Step 5. Give it a name, set up the file system to be FAT32 or exFAT, and keep other information as default.
Step 6. Check the Perform a quick format option and click OK.
Solution 2. Use an adapter with a power supply
If you have an external hard drive with spinning platters, you'll need to connect it to a separate power source to use it with your iPad.
Alternatively, you can opt to buy a plug-in powered USB-C hub for your iPad Pro. This hub should have at least two available ports: one for data transfer to your external hard drive and another for supplying additional power to the external device while also charging your iPad device.
I wish you the best of luck!