The invention may be embodied in a SAS expander with register bits within Phys associated with I/O devices. Setting and unsetting the register bit in the Phy associated with a particular physical or logical device allows I/O traffic to be blocked and unblocked, as desired, to the selected physical or logical devices. In a particular embodiment, when the register bit is set to a blocking state, an OPEN request that comes in on the SAS link is rejected using OPEN_REJECT (RETRY). Phy register bits may be provided for multiple different SAS protocols that can be controlled individually for each attached SAS device. The Phy register bit may also be used to reject only SAS connection requests that attempt to leave a particular SAS link.