block device
also: storage device, disk device
A hardware device that stores data in fixed-size blocks and allows random read/write access, such as hard drives, SSDs, and USB drives. Block devices are accessed through special device files in /dev.
A block device is any storage device that organizes data into fixed-size chunks called blocks (typically 4096 bytes) and permits random access to any block without reading sequentially. This differs from character devices, which process data as a stream.
Block devices appear as files in the /dev directory with a 'b' flag. Common examples include /dev/sda (entire disk), /dev/sda1 (disk partition), and /dev/nvme0n1 (NVMe SSD). You cannot directly read from these files; instead, you create filesystems on them and mount them.
Example: sudo mkfs.ext4 /dev/sda1 creates an ext4 filesystem on the first partition of the first SATA disk, then sudo mount /dev/sda1 /mnt makes it accessible. The kernel buffers block I/O operations for efficiency.