Microsoft SQL Server uses Microsoft Windows operating system input/output (I/O) calls to perform read and write operations on your disk. SQL Server manages when and how disk I/O is performed, but the Windows operating system performs the underlying I/O operations. The I/O subsystem includes the system bus, disk controller cards, disks, tape drives, CD-ROM drive, and many other I/O devices. Disk I/O is frequently the cause of bottlenecks in a system.
Monitoring disk activity involves two areas of focus:
Monitoring Disk I/O and Detecting Excess Paging
Isolating Disk Activity That SQL Server Creates
For more information see, Monitoring Disk Usage