You would model the drive slot and the drive together. An empty drive slot would appear as a member of the Drive collection with an "Absent" state (in Status), and very few other properties. The slot number and related details can be shown in the Location object (for a drive and for the slot).
Is there some other status/state that you'd need to report separately?
How to represent multiple LEDs on one Chassis since there is only one IndicatorLED in the Chassis schema definition.
We don't want to represent the LEDs themselves, but rather the information that they convey. The "IndicatorLED" property is shown as that is the function - to provide a visual identifier to a user (location / identifier). Other LEDs that show the status of a drive or it's power on/off - should be conveyed through the Status object properties. This allows software to obtain the underlying information with better detail than is typically made available (and not in a consistent manner!) with simple LED on/off/blinking information.