Started doing a surface scan with Tech Tool Pro on an external drive connected to the MacBook Pro which, as an older model, has USB 2, FW 800 or Thunderbolt connections. The Thunderbolt drive interface is in use on the MBA, & a backup drive is in the FW port. TTP reported the scan was going to take over 2 hours.
Out comes the secret weapon: a Kanex Thunderbolt to Gigabit Ethernet/USB 3 adaptor.
With the drive connected via that interface, the test time was cut to 20 minutes.