Apple has confirmed that it reserves the right to change the supplier and specification of the hard drives built into the latest iMac models, announced last week. In MacUser’s tests, undocumented variations between the drives fitted in different models resulted in transfer speeds ranging from 76 to 164 megabytes per second (MB/sec), making the quickest units twice as fast as the slowest.
The performance gap is invisible in Apple’s published specifications and is not guaranteed to be preserved in future production runs, meaning buyers can’t be sure what results they’ll see from the hardware they receive.
The 21.5in iMac supplied by Apple for testing, corresponding to the £999 configuration available from the Apple Store (2.5GHz Core i5), was fitted with a 500GB WD Caviar Blue drive (WD5000AAKS). In MacUser’s tests, this delivered an average transfer rate of 76MB/sec, peaking at 85MB/sec.
The supplied 27in model, as sold at £1399 (2.7GHz Core i5), contained a WD Caviar Black (WD1001FALS) 1TB drive which performed ahead of the manufacturer’s speed claims in our tests, achieving an average transfer rate of 164MB/sec and peaking at 173MB/sec. No details of the drives used are given in Apple’s published specs, which list only the same rotational speed of 7200rpm for all models.
MacUser researched additional iMac units and consistently found the same two drives used in the respective models. Among other configurations, we found the same 1TB WD Caviar Black unit in the 21.5in 2.7GHz Core i5 iMac, where it achieved average and maximum write speeds of 157MB/sec and 167MB/sec.
However, we found an entirely different drive in several 3.1GHz Core i5 models and in one example of the top-end 3.4GHz Core i7. This 1TB Seagate Barracuda 7200.12 unit (ST31000528AS) claims a sustained maximum transfer rate of 125MB/sec, but our tests showed average and maximum random write speeds of 101MB/sec and 111MB/sec: about one-third faster than the Caviar Blue, but one-third slower than the Caviar Black – which we also found in another 3.4GHz Core i7 iMac, nominally of the same specification.
Various factors account for differences in hard disk performance, including the amount of cache fitted (the WDC Blue drive had 16MB, while the faster WDC Black and Seagate had 32MB) and the number of platters used.
In disk-heavy tasks, these substantial differences will noticeably affect overall performance. In general use, paging apps and data in and out of RAM – a common background process for Macs with several heavyweight apps in use at once, especially with relatively small amounts of memory, such as the 4GB fitted by Apple as standard – will also cause more delays if the hard disk is slower.
Apple’s choice of drives doesn’t appear to correspond to the performance level of the system, so higher-priced models may have a slower drive. The variation between our two Core i7 models indicated that orders for the same specification could be delivered with different drives.
A spokesperson for Apple told MacUser: “As always we do not specify the HDD vendors, which allows us to get the best deal for the customer.” Customers may feel that they would prefer to know what deal they were getting before handing over their cash.
