rear USB ports ONLY
Physically, the rear ports tend to be sat on the motherboard and so are known to be 2.0-compliant. The front ports are generally connected to the motherboard header by wiring which can sometimes fall out of spec.
Seagate specifies "rear ports" because on some older systems the front ports are 1.1 and the customer doesn't realise this, and on some systems they're 2.0 but cause problems with fast devices (see above). Finally, on some motherboards only the rear ports are root hubs, so anything sensitive to not being connected to a root hub will have problems with the front ports anyway. Finally finally, since the rear ports are root hubs, they are not sharing bandwidth so the external drive can perform as advertised.