SanDisk 2TB SSD will not mount on iPad Pro 2nd Gen

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.

If your iPad Pro has a Lightning port, 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 - 20W providing a larger margin to power the connected storage device.

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.

Have you enabled any SanDisk-proprietary security or encryption features on the SSD - a process usually performed on a Windows PC or Mac Computer? If so, the feature may not be compatible with iPadOS (unless there is a manufacturer-provided App that must be installed on the iPad).