A single host adapter hardware I/O control block contains information used to specify a transfer of data from a host system to a first target device and in addition information that specifies whether the data is to be mirrored, and if so, optionally identifies a second target device on which the data is to be mirrored. After transferring the single hardware I/O control block to the host adapter integrated circuit, the host adapter integrated circuit determines whether the hardware I/O control block specifies a mirrored transaction. If a mirrored transaction is specified, the host adapter integrated circuit generates a second hardware I/O control block for the second target device using the information in the first hardware I/O control block. When the execution of both hardware I/O control blocks is complete, the host adapter integrated circuit provides a single completion notification to the host system.