I believe you are confusing HDD's (mechanical hard drive) with SSD's (solid state hard drives). As far as I know, there aren't any 20TB SSD's in existence unless it's a rare enterprise model and they would be extremely cost prohibitive (like, 10's of thousands of dollars for that capacity).
Anyways, if you're looking for large capacity drives, you can either buy a Western Digital external and use it's native enclosure or I always recommend https://serverpartdeals.com/collections/hard-drives as a place to shop for high capacity drives at a decent price. Do note, I linked to the HDD page. You can find SSD's at this link for price reference: https://serverpartdeals.com/collections/solid-state-drives
As an additional point of reference, I use the Seagate EXOS X16 and 18's here for my Plex and backup drives. https://serverpartdeals.com/collections/hard-drives?sort=price-ascending&pf_t_brand=brand%3ASeagate&pf_t_brand=brand%3AHGST&pf_t_brand=brand%3AToshiba&pf_t_brand=brand%3AWestern+Digital&pf_t_capacity=capacity%3A18TB&pf_t_capacity=capacity%3A16TB&pf_t_capacity=capacity%3A14TB&pf_t_interface_type=interface%3ASATA