Loss of Firewire connection on iMac mid 2010 running El Capitan

I have a PreSonus Firebox I use to record directly into my iMac or Macbook Pro - both are mid-2010 and both recently upgraded to El Capitan. Although the Firebox is listed as legacy by PreSonus, it is recognized and is functional on my MBP but is not recognized on the iMac. It is listed under "Firewire" on the 'System Report" under "Hardware" on the MBP, but not the iMac. Is there an explanation for the disparity? Any thoughts on how to address and perhaps force a Firewire recognition on the iMac?