No, there is no write cache on the S100 RAID controller:
All SATA disks have a small amount of cache to help improve performance. In most cases, this cannot be turned on/off without a special interface (tool directly from the drive manufacturer - WD, Seagate, etc. - or the BIOS). If you cannot turn it off in the BIOS, then 1) you might not be able to edit the drive's ability to use the write-cache, or 2) it might not even be used at all (on-disk write cache is ignored by PERC 6/i and H700 controllers). If you can't turn off the cache at the disk level (if it is even being used), then it is no worse than a desktop losing power.