The SMART attribute ID 194 (Temperature_Celsius) often causes confusion when it reports values that contradict physical observations. In your case, while the drive feels cool at 70°F ambient temperature, SMART shows:

194 Temperature_Celsius     0x0002   150   150   000    Old_age   Always       -       40 (Min/Max 20/49)

The key components in this output are:

• VALUE/WORST (150): Normalized values (manufacturer-specific scaling)
• RAW_VALUE (40): Actual temperature in Celsius
• THRESH (000): Indicates no threshold for warning

Different manufacturers implement SMART attributes differently. Western Digital drives, for example, often report temperature as:

VALUE = 255 - (actual_temp_in_C * scaling_factor)

While your Hitachi drive shows direct correlation between VALUE and RAW_VALUE.

To confirm actual temperature, use alternative tools:

# hddtemp /dev/sdb
/dev/sdb: Hitachi HDS722020ALA330: 40°C

# sudo smartctl -A -d sat /dev/sdb | grep -i temp
194 Temperature_Celsius     0x0002   150   150   000    Old_age   Always       -       40

While 40°C is within normal operating range (most HDDs tolerate 0-60°C), monitor these patterns:

• Sudden spikes >10°C without load changes
• Consistent readings above 55°C
• Discrepancies between multiple monitoring tools

For proactive monitoring, create a cron job with this script:

#!/bin/bash
THRESHOLD=50
TEMP=$(smartctl -A /dev/sdb | awk '/194/{print $10}')

if [ "$TEMP" -gt "$THRESHOLD" ]; then
    echo "WARNING: Drive temperature $TEMP°C exceeded threshold" | mail -s "HDD Alert" admin@example.com
fi

Common causes for inaccurate readings:

• eSATA/SATA controller firmware bugs
• Drive sleep state misreporting
• SMART tool version incompatibilities (especially with older CentOS 6.x)

---

When SMART reports Attribute 194 (Temperature_Celsius) showing 150 while the physical drive feels cool and raw_value displays 40, we're dealing with a common interpretation issue in smartmontools. The confusion stems from how different manufacturers implement SMART attributes.

Western Digital drives often use this particular encoding scheme where:

Reported value = (100 - actual_temp) + calibration_offset
Raw_value = actual temperature in Celsius

In your case, the raw_value of 40°C makes perfect sense for an eSATA drive in a 70°F (21°C) environment.

To get the true temperature reading, always check the RAW_VALUE column:

# Alternative commands for verification:
hdparm -H /dev/sdb
smartctl -A /dev/sdb | grep -i temperature

The raw value of 40 matches physical observations, while the normalized value of 150 is meaningless in this context.

For proper monitoring scripts, always parse the raw value:

#!/bin/bash
TEMP=$(smartctl -A /dev/sdb | awk '/Temperature_Celsius/ {print $10}')
echo "Actual drive temperature: ${TEMP}°C"

This prevents false alerts from normalized values.

The older smartctl version (5.43) in CentOS 6.6 doesn't automatically decode manufacturer-specific implementations. Newer versions (6.0+) include better vendor-specific interpretation:

# On modern systems:
smartctl -d sat -A /dev/sdb

The '-d sat' flag helps with proper SATA/SAS interpretation.

Legitimate temperature warnings would show:

• RAW_VALUE exceeding 55°C for consumer drives
• Consistent upward temperature trends
• Correlated with other SMART warnings

Your drive shows none of these red flags.

For proper enterprise monitoring, use this Nagios-compatible check:

#!/bin/bash
WARN=50
CRIT=60
TEMP=$(smartctl -A /dev/$1 | awk '/Temperature_Celsius/ {print $10}')

if [ $TEMP -ge $CRIT ]; then
  echo "CRITICAL: Drive $1 at ${TEMP}°C"
  exit 2
elif [ $TEMP -ge $WARN ]; then
  echo "WARNING: Drive $1 at ${TEMP}°C"
  exit 1
else
  echo "OK: Drive $1 at ${TEMP}°C"
  exit 0
fi