lsblk gives an overview of the disks available on the system
Example
My NAS has 6 6TB hard-disks and 1 512MB NVMe
worker@nas $ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 5.5T 0 disk
├─sda1 8:1 0 2G 0 part
└─sda2 8:2 0 5.5T 0 part
sdb 8:16 0 5.5T 0 disk
├─sdb1 8:17 0 2G 0 part
└─sdb2 8:18 0 5.5T 0 part
sdc 8:32 0 5.5T 0 disk
├─sdc1 8:33 0 2G 0 part
└─sdc2 8:34 0 5.5T 0 part
sdd 8:48 0 5.5T 0 disk
├─sdd1 8:49 0 2G 0 part
└─sdd2 8:50 0 5.5T 0 part
sde 8:64 0 5.5T 0 disk
├─sde1 8:65 0 2G 0 part
└─sde2 8:66 0 5.5T 0 part
sdf 8:80 0 5.5T 0 disk
├─sdf1 8:81 0 2G 0 part
└─sdf2 8:82 0 5.5T 0 part
nvme0n1 259:0 0 465.8G 0 disk
├─nvme0n1p1 259:1 0 512M 0 part /boot/efi
├─nvme0n1p2 259:2 0 464.3G 0 part /
└─nvme0n1p3 259:3 0 976M 0 part [SWAP]