Read this recent post HERE .
The basics: you want USB3 speeds if possible from your card reader, and you won't get it from that slow old device. The post shows you how to get USB3 speeds. The firewire and the USB ports on your card reader both go at about USB2 speeds.... 1/10th that of USB3.
There is only one set of header pins along the bottom edge of your motherboard for firewire, and it is the slow interface of firewire 400 that about matches speed of USB2. That header already is occupied with the front firewire 400 cable that feeds to the front of your Z420.... regardless of what cable you choose to use it will still be only firewire 400. The USB2 cable from your USB2 card reader plugs into any one of the 3 spare internal USB2 sets of header pins.... also stuck at 1/10th speed the speed of USB3.
Read the post.... works great. Might require some extra parts and reading the manual but worth it. Put that HP "2x2" Texas Instruments chipset card in the very top short PCIe slot in the Z420/Z620 workstations. That will get you full USB3 speeds......