In Linux systems, /dev/sda represents the entire physical disk, while /dev/sda1 refers specifically to the first partition on that disk. This naming convention follows the Linux device hierarchy:

/dev/sdX   - Entire disk (X being a letter: a, b, c, etc.)
/dev/sdXN  - Specific partition (N being a number: 1, 2, 3, etc.)

You're absolutely correct in observing that partition devices (/dev/sda#) only appear under certain conditions:

• Unpartitioned disks: If a disk has no partition table (completely raw), only /dev/sda will exist
• Corrupted partition tables: When the partition table is damaged or unrecognizable
• Special disk formats: Some storage devices like certain SSDs or LVM physical volumes might not use standard partitioning

The ability to mount both devices depends on the filesystem implementation:

# Mounting a partition (most common case)
sudo mount /dev/sda1 /mnt/data

# Mounting the entire disk (rare but possible)
sudo mount /dev/sda /mnt/entire_disk

Modern Linux systems typically prevent mounting entire disks by default because:

1. Most filesystems expect a partition structure
2. Mounting the raw device could corrupt partition tables

Let's examine a disk's status using lsblk:

$ lsblk -f
NAME   FSTYPE LABEL UUID                                 MOUNTPOINT
sda                                                        
├─sda1 ext4   data  5a3a5b3c-4d2e-1f0a-9b8c-7d6e5f4a3b2c /mnt/data
└─sda2 swap         d8e7f6a5-b4c3-2d1e-9f8a-7b6c5d4e3f2a [SWAP]

To check if a device has partitions:

$ sudo fdisk -l /dev/sda
Disk /dev/sda: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Disk model: ST2000DM008-2FR1
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: A1B2C3D4-E5F6-7G8H-9I0J-K1L2M3N4O5P6

Device     Start        End    Sectors  Size Type
/dev/sda1   2048 3907028991 3907026944  1.8T Linux filesystem

While generally not recommended, you can work with raw disks in programming:

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

int main() {
    int fd = open("/dev/sda", O_RDONLY);
    if (fd == -1) {
        perror("Error opening device");
        return 1;
    }
    
    char mbr[512];
    if (read(fd, mbr, sizeof(mbr)) != sizeof(mbr)) {
        perror("Error reading MBR");
        close(fd);
        return 1;
    }
    
    // Process MBR data here
    
    close(fd);
    return 0;
}

• Always prefer mounting partitions (/dev/sda1) over raw devices
• Use lsblk or blkid to verify device structure before mounting
• For raw disk operations, consider using dd with caution
• When scripting, include checks for partition existence:

if [ -b /dev/sda1 ]; then
    echo "Partition exists, mounting..."
    mount /dev/sda1 /mnt/data
else
    echo "No partition found, checking raw disk"
    # Additional checks here
fi

In Linux systems, /dev/sda represents the entire physical disk, while /dev/sda1 refers to the first partition on that disk. The key differences:

# Display block devices
lsblk

# Typical output:
NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda      8:0    0  100G  0 disk
├─sda1   8:1    0   50G  0 part /
└─sda2   8:2    0   50G  0 part /home

Partition devices (/dev/sda#) only appear when:

• The disk has a valid partition table (MBR/GPT)
• At least one partition exists
• The kernel has detected the partition layout

Example of an unpartitioned disk:

# Output for unpartitioned disk:
NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda      8:0    0  100G  0 disk

Both devices can be mounted under specific conditions:

# Mounting the entire disk (rare use case)
sudo mount /dev/sda /mnt/rawdisk

# Mounting a partition (standard practice)
sudo mount /dev/sda1 /mnt/partition

Mounting the raw device (/dev/sda) only works when:

• The filesystem spans the entire disk (no partition table)
• For specialized operations like forensic analysis
• With certain filesystem types that don't use partitions (e.g., some LVM configurations)

To check partition status:

sudo fdisk -l /dev/sda
sudo parted /dev/sda print

Creating a new partition:

# Using fdisk
sudo fdisk /dev/sda
# Follow interactive prompts to create partition

# Using parted non-interactively
sudo parted /dev/sda --script mklabel gpt mkpart primary ext4 1MiB 100%

When working with raw devices vs partitions:

# Creating filesystem on entire disk
sudo mkfs.ext4 /dev/sda

# Creating filesystem on partition
sudo mkfs.ext4 /dev/sda1