Ever added a new disk to your server only to find it playing hide-and-seek with your system? Or perhaps you’ve hot-swapped a drive and now you’re wondering how to make Linux acknowledge its existence without the dreaded reboot? Well, grab your magnifying glass because today we’re becoming disk detectives!

The Case of the Invisible Disk

When you connect a new disk to your Linux system, sometimes it doesn’t show up automatically. This isn’t because your system is being stubborn—it’s just that Linux needs a little nudge to scan for new hardware.

Your Detective Toolkit

The command you shared is actually a powerful tool in our disk detection arsenal:

# Scan a specific host controller
echo "- - -" > /sys/class/scsi_host/host0/scan

# Or scan ALL host controllers (recommended)
for host in /sys/class/scsi_host/*; do
  echo "- - -" | tee $host/scan
  ls /dev/sd*
done

# Rescan a specific disk that's already detected
echo "1" > /sys/class/block/sdb/device/rescan

Breaking Down the Clues

What’s happening here? Let’s decode this like proper detectives:

The magic string "- - -" tells the system to scan all channels, all targets, and all LUNs (Logical Unit Numbers) on that host controller. It’s like telling your system, “Check everywhere, leave no storage unturned!”

The for loop visits each SCSI host controller in your system—these are the pathways to your storage devices. By scanning each one, we’re covering all possible connections.

After each scan, the ls /dev/sd* command shows us which disk devices are currently detected, giving us immediate feedback on our investigation.

The last command is for when a disk is already visible but might have changed—perhaps it’s grown in size after being expanded in a virtual environment or SAN. This tells the system to take another look at a specific disk.

When to Use These Incantations

• After physically adding a new hard drive
• After adding a virtual disk in a VM
• After resizing a disk in your virtual environment or SAN ↔️
• When a disk mysteriously disappears (it happens to the best of us!)

Success! Case Closed

If all goes well, your new disk should appear in the /dev/sd* list. Now you can format it, partition it, and bring it into your storage pool—ready to store more cat pictures , cryptocurrency wallets , or that novel you’ve been meaning to write!

Remember: Being a good disk detective means you can solve storage mysteries without the hassle of rebooting your system. Now go forth and detect those disks with confidence!