I probably spoke out of place, but if you have multiple I/Os going on your hard disk greatly reduces it's performance. Plus, you don't file transfer over TCP/IP if you are using windows you do it over SMB which has lots of overhead in itself, especially SMB 2 which was introduced in Vista. TCP/IP is the protocol for networking, while SMB is the actual network application that runs on the TCP/IP Application layer of the OSI model to share files, authenticate, and other things like printers or other network devices that support SMB. Back in the day SMB used to run over IPX networks, which was a considerably slower and inferior protocol to TCP/IP.
Every time you add a process to your computer that is reading or writing from the hard disk you are essentially cutting down on it's performance. Sure SATA drives can transfer speeds up to 150MBs (that is bytes) per a second over the cable, but that does not mean that your hard drive will be able to output or input that to the disk from that speed.
When you get into large networking, like SANs and stuff like that that run over fiber disk I/O is the bottle neck. Now, if you are doing a simple peer to peer network, and only doing 1 file transfer, well disk I/O will probably not come into play as much as I was implying. However, having a faster hard disk, or say running an array (RAID) will increase data throughput.
