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Note that Intel Wireless cards are meant to be installed by Original Equipment Manufacturer (OEM) technicians on certified systems only, no retail versions of the Intel Wireless Adapters are offered to the public.
Only Computer manufacturers such as Toshiba*, Dell*, Acer*, Lenovo*, etc, have the proper ways to integrate these adapters, since the systems must pass a number of certifications and regulatory approvals and can choose which hardware can be used in accordance to the certifications obtained for a particular computer model.
Also some manufacturers might block at BIOS level the usage of unauthorized hardware, so it is true that the adapter might not be compatible.
Try checking with Lenovo* to see if they have a different card for the E420 system, maybe some other model that they offer might suit your needs.
For additional information please check the following links:
Intel® WiFi Products - Upgrade or exchange Intel® wireless adapter hardware
Intel® WiFi Products - Why doesn't my laptop recognize my new Intel wireless adapter?