I have a new SSD as a cache drive, and just started to get alerts (had them for 2 days so far)
First alert is:
Event: unRAID Cache disk SMART health [197]
Subject: Warning [NAS] - current pending sector is 1
Description: CT250MX500SSD1_1803E10AC5FE (sdf)
Importance: warning
then - 20 mins later, a 'clear' alert
Event: unRAID Cache disk SMART message [197]
Subject: Notice [NAS] - current pending sector returned to normal value
Description: CT250MX500SSD
It's a problem with the MX500, likely a firmware bug, I have 12 in a pool and keep getting the same warning, 1 pending sectors that disappears after a few minutes, hope they fix it with a firmware update.