Western Digital developed a technology to increase media format efficiency, thus enabling larger drive capacities. Called as “Advanced Format,” reduces amount of overhead for each data sector by pooling together information. Instead of requiring a single Sync/DAM header and ECC (error correction) block for each sector, it would create much larger 4KB sectors that use one set of each header. The format requires a larger ECC block but overall provides significantly more data in a given drive. On average, WD expects a fully formatted drive to get between 7 and 11 percent more usable storage than with traditional formatting. Some models of the WD Caviar Green are built using Advanced Format. Over time more models and capacities will be added. WD drives with Advanced Format include special installation information on the drive label so be sure to read the label on your drive before installing it.
More info: Advanced Format Hard Drive