"Device is not ready"

I am trying to recover files from a damaged hard drive. I have attached the drive using a USB3 caddy to a Windows 10 PC. The disk shows in File Explorer as having two partitions, which are assigned H: (Windows) and I: (LENOVO)

When I point Recuva at H: and press Scan, it responds by saying the drive is not ready. It is able to scan I: and that is proceeding as I write.

What is the issue with the H: partition?