Micro SD: listing disks, writing ISOs and secure erasing

Writing a system image to an SD card or USB drive from the terminal is simpler than it looks — and faster than opening Etcher most of the time.

Identify the correct disk

Before anything else, identifying the right device is critical. dd doesn’t forgive mistakes:

lsblk -p

The -p flag shows the full device path (e.g., /dev/sda, /dev/mmcblk0). Look for the size matching your SD card.

Write an ISO directly

sudo dd if=linux-2020.4-live-amd64.iso of=/dev/sda bs=4M conv=fsync status=progress && sync

• if= → input file (the ISO)
• of= → output device (the SD card — without partition number: /dev/sda, not /dev/sda1)
• bs=4M → transfer block size (4MB is a good balance)
• conv=fsync → ensures data is physically written
• status=progress → shows real-time progress
• && sync → forces synchronization before finishing

Write a .xz compressed ISO

Raspberry Pi images usually come compressed in .xz. You can decompress and write in a single command:

sudo xzcat linux-2020.4-live-amd64.iso.xz | dd of=/dev/sda bs=32M conv=fsync status=progress && sync

Or more generically:

xzcat ~/Downloads/<file.xz> | sudo dd of=<device> bs=32M

If you prefer to decompress first:

xz -d server-arm64+raspi.img.xz

This generates the .img file which can be written normally with dd.

Securely erase the disk (zero fill)

To erase a disk before discarding or reusing it:

sudo shred -n 2 -z -v /dev/sda1

• -n 2 → makes 2 passes with random data
• -z → makes a final pass with zeros (disguises the shredding)
• -v → verbose, shows progress

Warning: for SSDs and flash storage (like SD cards), shred doesn’t guarantee complete erasure because of wear leveling. For these cases, a single dd if=/dev/zero is already sufficient for practical purposes.