7200 RPM is the rotational speed, it doesn't have any direct relationship to MB/s throughput.
That depends on
-the hard drive (its design, areal density, seek performance, caching performance),
-the operating system,
-the size of the platters,
-how full the drive is,
-what interface the drive is using
-the type of data being read and written.
Neither does the cache size directly relate to MB/s
In general, a 7200 RPM drive will have higher throughput than a 5400 RPM drive. A desktop (3.5") drive will have higher throughput than a laptop (2.5") drive at the same rotational speed.
12 MB/s write and 27 MB/s read is pathetic performance for a hard drive, It is average- to decent-performance for a USB memory stick.
A typical hard drive is capable of up to 80 - 100 MB/s throughput when new. Drives slow down as they fill up. The fastest hard drives can hit about 120 MB/s. RAID arrays can get up tp 200 - 400 Mb/s (yes, I know that SATA-II and SATA-III interfaces are capable of up to 3.0 (300 MB/s) and 6.0 Gb/s (600 MB/s) but that is the *Interface* maximum, not the actual drive performance)
A SSD drive can manage up to 275 Mb/s
The USB 2,0 interface limits the performance of any storage device to about 30 - 40 MB/s
Firewire 400 is faster, Firewire 800 faster than that (about 80 MB/s), eSATA at 150 MB/s is the fastest common external interface, it can handle pretty much the full output of any single hard drive.
Keep in mind that the rates quoted are maximum rates -- average real-world transfer rates are affected by many factors and will be lower.