SQL Server 2008 R2 high Buffer I/O causes
If you look carefully at the waits graph, the coloured bars actually indicate lock waits, not I/O waits.

Also, the I/O graph shows increased system disk usage, not SQL Server disk usage.

Given that the memory graph is approaching your system memory size as the system utilization increases over time, I believe your instance is overcommitted for the current max server memory setting (or the max server memory setting isn't set at all), and one or more of the SQL Server memory pools are swapping to disk.

The solution is to lower the max server memory setting (or set it to an appropriate value if it isn't set). Please see my answer here for a more in-depth answer from a similar scenario, and my blog post:

Is my SQL Server's memory over-committed?