Most of the users does not know the Slow I/O could cause the SharePoint performances. The poor disk performance can cause the major impact on the SharePoint performance. One of the troubleshooting method would be use a Performance Monitor with counter measures to get a good picture of health of the disk I/O subsystem and any latency SQL Server is experiencing from its poor performance.
Microsoft had released a tool called “SQLIO Disk Subsystem Benchmark Tool” which can also be used to determine the I/O capacity of a given configuration. Also, you can use a tool such as IOMeter or sqlio.exe–both of which are freely downloadable–to benchmark the I/O performance of a disk.
You can download it from here: http://www.microsoft.com/downloads/details.aspx?familyid=9A8B005B-84E4-4F24-8D65-CB53442D9E19&displaylang=en
For more information: http://support.microsoft.com/kb/231619
Microsoft Engineer had posted a blog entry on Troubleshooting the Slow I/O in SQL Server. You can check his blog entry here: http://blogs.msdn.com/b/askjay/archive/2011/07/08/troubleshooting-slow-disk-i-o-in-sql-server.aspx