Why is no temperature displayed for my external FireWire or USB hard disk?

Nearly all FireWire or USB hard disks are in fact SATA hard disks accessed via a “FireWire/USB to SATA bridge device”. Those bridge chips only translate a subset of the ATA commands between the FireWire/USB and the ATA protocol standards. In particular, commands to transfer S.M.A.R.T. diagnostic data (which are needed to access the temperature sensors inside the drives) are usually not supported. For this reason it is technically impossible to transport temperature data across a bridge. There are technical workarounds for this problem, but Apple currently does not support them in macOS.

Use the menu item Window > Show Drive Overview or press ⌘ + 8 to display the transport protocols of all disk drives and their S.M.A.R.T. capabilities.

Further information: Displaying System Information