03-19-2009 05:10 PM
03-20-2009 08:23 AM
None. Drives are normally marketed indicating the 'interface speed' which is much higher than the speeds to read/write from/to the platters. However, if you look at the specifications it is quite common to cite also the 'max read speed' or 'max write speed' or 'disk transfer speed' which are closer and normally taken at the outer tracks, where the disk is faster. Tools such as hdtach or hdtune can easily give the complete curves and calculate the average speed. Note also that the 'access time' or 'seek time' (access time=seek time+rotational delay) is also a very important factor to determine the drive's speed.
03-24-2009 03:13 PM - edited 03-24-2009 03:16 PM
The product manual for my 120GB drive uses the Outside Diameter for its max data rate spec. I'd be looking at the "sustained" data transfer rate rather than the "burst" rate. I expect that the external data rate is based on 512 bytes per sector whereas the internal data rate may include the ECC bytes, sector ID bytes and other overhead. The I/O data rate probably reflects the speed of the interface, ie the speed at which data can be transferred between the motherboard and the HD's cache RAM.
Barracuda 7200.7 Plus Product Manual:
Spindle speed (RPM) (± 0.2%) 7,200
Internal data-transfer rate OD (Mbytes/sec max) 85.4
Sustained data transfer rate OD (Mbytes/sec max) 58
I/O data-transfer rate (Mbytes/sec max) 100 (Ultra DMA mode 5)
You could calculate the theoretical maximum transfer rate, in MB/s as follows:
sectors/track x 120 revs/sec x 512 bytes/sec / 1024 / 1024
According to my measurements ...
... the number of sectors/track on the OD is somewhere between 0x42F and 0x52F.
This gives a max transfer rate of between 63 and 78 MB/s.