One SSD RAM disk in a pool is showing "unavailable" in the pool status and causing the pool to be in a "degraded" state.
The SSD RAM disk is also missing from the device list.
This symptom can mean the SSD RAM disk has failed and needs to be replaced or possibly just needs to be reset.
Before replacing the RAM drive the first step should be to try and bring the drive back online by pulling the drive out of the slot and leave it out for 60 minutes to let the capacitor drain and allow to drive to completely reset.
After 60 minutes you can re-seat the card and verify if the drive is able to be rediscovered.
If the drive is rediscovered and is available the pool will begin to use it again and the degraded pool state should be corrected.
Please review the link below for additional details: