Disks present but SMART is unreadable

One or more fixed disks are present but their SMART health cannot be read, so their health is invisible to monitoring. This is NOT a drive fault: it is a coverage gap. The usual cause is that smartmontools (the `smartctl` binary) is not installed, or a disk sits behind a RAID/HBA controller that needs a specific `smartctl -d` device type. While SMART is unreadable, a real failure on the affected disks would not raise smart_failing or wear alerts. Fix by installing smartmontools and confirming each disk reads with `smartctl -H`, or by determining the controller's device type (`smartctl --scan-open`). Acknowledge this advisory if a listed device genuinely exposes no SMART (some virtual or enclosure devices).