HDD 3.5 pockets
prices on 421 model3,5"
— pockets designed for 3.5" drives, while some models in this category also allow the installation of 2.5" drives; also included are adapters for using laptop 2.5" media in desktop cases with 3.5" slots.Actually, 3.5" is just the traditional form factor for internal drives for full-sized desktop PCs. Accordingly, internal pockets for this form factor are used exclusively in PCs or servers; they are too bulky for laptops; moreover, most of these pockets are a "sled" chassis (see "Destination — for PC"). External solutions turn out to be more bulky than models under 2.5", however, due to the absence of strict restrictions on the size, capacious drives for such pockets are much cheaper than miniature analogues of the same capacity.
Also note that most 3.5-inch drives are just traditional hard drives (or hybrid SSHD devices), SSD modules in this form factor are practically not produced. And the connection of such drives (including to pockets) is carried out via the SATA interface; its speed can reach 600 Mbps, which is quite enough for modern HDDs.