How to detect new hard disk attached without rebooting?

As was mentioned above, you could scan all existing hosts with a one-liner:

for host in /sys/class/scsi_host/*; do echo "- - -" | sudo tee $host/scan; ls /dev/sd* ; done

and the result:

$ for host in /sys/class/scsi_host/*; do echo "- - -" | sudo tee $host/scan; ls /dev/sd* ; done
- - -
/dev/sda  /dev/sda1  /dev/sda2  /dev/sdb  /dev/sdb1  /dev/sdc  /dev/sdc1
- - -
/dev/sda  /dev/sda1  /dev/sda2  /dev/sdb  /dev/sdb1  /dev/sdc  /dev/sdc1

    ︙

- - -
/dev/sda  /dev/sda1  /dev/sda2  /dev/sdb  /dev/sdb1  /dev/sdc  /dev/sdc1
- - -
/dev/sda  /dev/sda1  /dev/sda2  /dev/sdb  /dev/sdb1  /dev/sdc  /dev/sdc1
- - -
/dev/sda  /dev/sda1  /dev/sda2  /dev/sdb  /dev/sdb1  /dev/sdc  /dev/sdc1  /dev/sdd  /dev/sdd1

The last line shows us /dev/sdd device was discovered.

Below is the command that you need to run to scan the host devices so it will show the new hard disk connected.

echo "- - -" >> /sys/class/scsi_host/host_$i/scan

$i is the host number

This worked for me to refresh all devices: (As an easier to run command)

echo "- - -" | tee /sys/class/scsi_host/host*/scan