Change disk to HDD for clarity (donnemartin#295)

README.md

@@ -1613,9 +1613,9 @@ Read 4 KB randomly from SSD*           150,000   ns      150 us          ~1GB/se
 Read 1 MB sequentially from memory     250,000   ns      250 us
 Round trip within same datacenter      500,000   ns      500 us
 Read 1 MB sequentially from SSD*     1,000,000   ns    1,000 us    1 ms  ~1GB/sec SSD, 4X memory
-Disk seek                           10,000,000   ns   10,000 us   10 ms  20x datacenter roundtrip
+HDD seek                            10,000,000   ns   10,000 us   10 ms  20x datacenter roundtrip
 Read 1 MB sequentially from 1 Gbps  10,000,000   ns   10,000 us   10 ms  40x memory, 10X SSD
-Read 1 MB sequentially from disk    30,000,000   ns   30,000 us   30 ms 120x memory, 30X SSD
+Read 1 MB sequentially from HDD     30,000,000   ns   30,000 us   30 ms 120x memory, 30X SSD
 Send packet CA->Netherlands->CA    150,000,000   ns  150,000 us  150 ms
 
 Notes
@@ -1627,7 +1627,7 @@ Notes
 
 Handy metrics based on numbers above:
 
-* Read sequentially from disk at 30 MB/s
+* Read sequentially from HDD at 30 MB/s
 * Read sequentially from 1 Gbps Ethernet at 100 MB/s
 * Read sequentially from SSD at 1 GB/s
 * Read sequentially from main memory at 4 GB/s