Nick,
The cache memory available on Intel® RAID controllers can improve the overall system performance. Writing data to the controller's cache memory is much faster than writing it to a storage device. Write operations appear to complete very quickly at the software application level. The Intel® RAID Controller writes the cached data to the storage device when system activity is low or when the cache is getting full. The risk of using write-back cache is that the cached data can be lost if the AC power fails before it is written to the storage device. This risk factor is eliminated when the Intel® RAID Controller has a battery back-up unit (BBU) installed.
SSD Cache uses Solid-State Devices as secondary cache and is designed to improve the read performance of the Intel RAID Controller. SSD Cache doesn't provide any data protection per sey. See the SSD Cache 2.0 Feature Overview for more information.
Data Protection is another matter. As noted above, the risk of using write-back cache is that the cached data can be lost if the AC power fails before it is written to the storage device (the virtual disk). The BBU powers the cache memory for a time to allow the cached data to be written to the VD with the power gone. That's the extent of of the data protection when using write-back cache.
Regards,
John