This article demonstrates how to replace a degraded multipath disk using the FreeNAS or TrueNAS web interface. Multipath disks are connected through multiple controllers. Multipath data paths provide redundancy. If one path fails, the disks are still accessible through other paths.
Both TrueNAS and FreeNAS operating systems support multipath hardware. When multipath disks are connected, the View Multipath button is added to Storage -> Volumes.
The Alert icon, located in the upper right of the web interface, flashes red when a disk in a multipath configuration degrades.
Click the Alert icon and a critical warning message indicates which multipath disk is degraded. Disk errors tend to be shown as single multipath errors, like the example shown here. These are the most common type of problem. Hardware problems with controllers or cables are rare, and usually show as multiple degraded multipaths.
Follow these steps to replace a degraded multipath disk.
1.) Click Storage -> View Multipaths to check the status of the multipath disks.
In this example, multipath/disk12 is DEGRADED. One of the paths to disk12 has failed.
2.) If the disk will be returned to the vendor, retrieve the model and serial number.
2.1 Click the Shell icon in the menu tree.
2.2 Enter this command in the Shell console, replacing the active disk name.
smartctl -a /dev/da11 | head -15
In this example, /dev/da11 is the active disk name.
3.) On a TrueNAS system, the physical location of the degraded disk can be identified by flashing its LED light.
3.1. Click Storage -> View Enclosure.
3.2 To activate the identifier LED, click the degraded disk’s Fault button.
3.3 The message Slot  has been successfully identified appears at the top of the screen.
3.4 The fault LED for the degraded disk will light up or blink. Use the light to identify the disk in the chassis.
3.5 If the disk has completely failed, the LED might not work. Turning on the fault LEDs on the disks immediately before and after the failed drive will help to locate it.
3.6 After the disk has been located, click Reset LED to turn off the LED fault light.
3.7 The message Slot [ ] has been successfully cleared appears.
3.8 Physically replace the degraded disk.
4.) Inform the web interface of the replaced disk.
4.1 Click Storage -> Volumes. Select the volume containing the multipath, then click the Volume Status icon.
4.2 Select the failed multipath disk by clicking it. A Replace button appears at the bottom of the screen. Click it to replace the disk.
4.3 Use the Member Disk dropdown menu to select the device name of the new disk. Click Replace Disk.
4.4. The message Disk replacement has been initiated appears.
5.) To verify the status of the new disk, click Storage -> View Multipaths.
The multipath disk status is now OPTIMAL instead of DEGRADED.
6. To verify the volume status is HEALTHY, in the graphical interface, click Storage -> Volumes.