I can't find a specific Apple KB reference, but the consensus seems to be that you need Firewire, Thunderbolt or USB-C; USB 2 doesn't appear to be supported.
From the MacSales blog - Mac OS 101: How to Boot and Use a Mac in Target Disk Mode
What You Need
The list is short, but essential.
- Two Macs. That may seem obvious, but it makes sense to point out that Target Disk Mode only works between two Macs; you can’t chain multiple Macs together. All of the connection types (FireWire, Thunderbolt, and USB-C) support hot connecting, meaning you can connect a cable between the two Macs while they are powered on. We recommend shutting down both Macs before proceeding, however.
- An appropriate cable to make the connection. Ideally, you should connect similar ports; that is FireWire to FireWire, Thunderbolt to Thunderbolt, or USB-C to USB-C. There are, however, exceptions. Using adapters to connect Thunderbolt to FireWire will usually work, as will Thunderbolt to USB-C. But not all adapters are known to work correctly in Target Disk Mode, so if you can, connect directly to the same port type.