To underscore Erik's comment, i don't see mass storage having a select.

The channel looks out not in.

If mag disk connects to the central processor then it's going to want to pour data into the host.

Assuming intelligence for the mass storage sub-element relieves the host of disk format overhead and real-time response, not necesarily throughput (unless DMA).

I would allocate an interrupt for mass storage support.

regards,
p