caching USB

sda is a regular SATA disk and sdd is a USB attached SATA disk. Why isn't sdd cached? # dd if=/dev/sdd of=/dev/zero bs=1024k count=100 100+0 records in 100+0 records out 104857600 bytes (105 MB) copied, 3.00316 s, 34.9 MB/s # dd if=/dev/sdd of=/dev/zero bs=1024k count=100 100+0 records in 100+0 records out 104857600 bytes (105 MB) copied, 2.98833 s, 35.1 MB/s # dd if=/dev/sda of=/dev/zero bs=1024k count=100 100+0 records in 100+0 records out 104857600 bytes (105 MB) copied, 1.29493 s, 81.0 MB/s # dd if=/dev/sda of=/dev/zero bs=1024k count=100 100+0 records in 100+0 records out 104857600 bytes (105 MB) copied, 0.0669201 s, 1.6 GB/s -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/