NCM runs its own Free Space Detector over SNMP to verify that sufficient space is available to perform the operation.
When a device like ASA 5505 doesn't answer the OID: 1.3.6.1.4.1.9.9.10.1.1.2.1.7 the tool displays the message "Free space cannot be automatically determined, please verify manually" requiring interaction, which is a pain if you are performing several upgrades.
With the option of bypassing the detection over SNMP using a command such show flash | inc free would make more sense.