The SMART fields that actually matter

smartctl -a prints dozens of lines. Four matter:

• Reallocated_Sector_Ct — sectors already replaced from the spare pool. Non-zero and growing means replace the disk.
• Current_Pending_Sector — unreadable sectors waiting for remapping, more urgent than reallocated ones.
• Offline_Uncorrectable — sectors even an offline scan cannot fix.
• UDMA_CRC_Error_Count — interface errors, usually cable or backplane rather than the disk.

smartctl -H returns PASSED or FAILED against a generous threshold; PASSED is not health. Most cloud disks do not pass SMART through at all, and only provider metrics help.

Run smartctl -t long occasionally; the short test misses deeper problems.