That is a fixed limit locked in place by the combination of the computer’s BIOS and the drive’s ATA version. Problem is, when you combine the two, you get a real world limitation of 1,024 cylinders, 63 sectors, and 16 heads, which works out to 528 MB (504 MiB). The early ATA specification supports 65,536 cylinders of up to 255 sectors and up to 16 heads, for a theoretical maximum drive size of 127.5 GB. The original PC BIOS supports up to 1,024 cylinders of up to 63 sectors and up to 255 heads, giving a theoretical limit of 8.4 GB (7.8 GiB).

ATA-6 (Ultra ATA/100) and newer, including SATA, 144 PBįrom the beginning, accessible hard drive capacity has also been limited by the computer itself.

ATA-1 through ATA-5 (Ultra ATA/66), 137 GB.

ATA-6) arrived with 48-bit addressing in 2002, in theory supporting drives up to 144 petabytes (128 PiB), although it will be a long, long time before drive of anywhere near that capacity become a reality. That remained the limitation until Ultra ATA/100 (a.k.a. The first ATA specification increased that to 28-bit addressing to support drives up to 137 GB (128 GiB). The original IDE specification supported 22-bit logical block addressing, which supported a maximum drive size of 2.1 GB (2.0 GiB).

Because the intelligence to manage an IDE drive came from the computer’s CPU rather than a dedicated onboard processor, IDE was less expensive to build and use than SCSI, which is probably the chief reason it was quickly adopted in the cost conscious PC world. IDE has a drive controller integrated with the hard drive, no longer requiring a separate hard drive controller card. Today’s SATA drive interface is descended from Parallel ATA, which is descended from Western Digital’s IDE (Integrated Drive Electronics) interface – also released in 1986, the same year as SCSI. And by the time Apple was putting a 233 MHz G3 CPU in the Beige Power Mac G3 (late 1997), that overhead was insignificant. By the time CPUs hit 33 MHz (the clock speed of the PowerBook 150 and Performa 630, the first two Macs with IDE, both introduced in July 1994), not so much.

The SCSI processor handles all those housekeeping detail, and this is also true of FireWire.īack in the era of 8 MHz computers (the speed of the four earliest Macs and a couple later ones), a smart interface like SCSI made a world of difference.

With SCSI, the drive has its own processor, so the operating system only has to ask it for data, not know where it is stored. With “dumb” interfaces, the computer’s operating system has to know the physical location of information on the hard drive – which sector of which track of which side of which platter to access to launch an application or read a data file. That’s because SCSI is an intelligent interface that hides the physical details of the hard drive from the computer.

2 tb hard drive for imac