Can I connect my Max One Slim Drive to my iPad 9th gen?

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.

As the iPad has limited power capabilities, dependent upon the Storage Device, an external power source is often required. For HDD or SSD, an external power source is almost certainly required.

Your iPad9 has a Lightning port, for which 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 you also use a newer model iPad with a USB-C connector, you may need to utilise 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.

The external storage device will obviously require an appropriate USB3 cable to connect to the USB port of the Lightning to USB3 Camera Adapter, or USB hub if used.

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

Unfortunately, unlike the latest USB-C iPads, the Lightning connector does not support all external storage devices - there are special external storage devices that support it.