iPad 5th generation 12.9
My IPad 12.9 5th generation iPad Pro, OS 16.6.1 not seen SSD external hard drive.
iPad Pro, iPadOS 16
Question marked as
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.
As the iPad has limited power capabilities, dependent upon the Storage Device, an external power source is often required.
If you have one of the newer iPad Pro models, 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.
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