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Low End Mac Tech Journal

Maximum Hard Drive Size

Daniel Knight - 2014.04.06

Hard drive capacity is limited not only by how densely bits can be packed on a magnetic platter, but also by the number of sectors and tracks and drive surfaces in the drive itself and the number the computer’s operating system is designed to handle.

One benefit of Apple adopting SCSI for its hard drives way back in 1986 is that it doesn’t have a maximum drive size, unlike the IDE/ATA drives used in the PC world.*

Mac Plus

Mac Plus, the first with SCSI

That’s because SCSI is an intelligent interface that hides the physical details of the hard drive from the computer. 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. 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. The SCSI processor handles all those housekeeping detail, and this is also true of FireWire.

PowerBook 100 SeriesBack 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. 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. And by the time Apple was putting a 233 MHz G3 CPU in the Beige Power Mac G3 (late 1997), that overhead was insignificant.

Parallel ATA Size Limitations

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. IDE has a drive controller integrated with the hard drive, no longer requiring a separate hard drive controller card. 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.

The original IDE specification supported 22-bit logical block addressing, which supported a maximum drive size of 2.1 GB (2.0 GiB). The first ATA specification increased that to 28-bit addressing to support drives up to 137 GB (128 GiB). That remained the limitation until Ultra ATA/100 (a.k.a. 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.

Maximum Drive Capacity

  • IDE, 2.1 GB
  • ATA-1 through ATA-5 (Ultra ATA/66), 137 GB
  • ATA-6 (Ultra ATA/100) and newer, including SATA, 144 PB

Real World Limitations

From the beginning, accessible hard drive capacity has also been limited by the computer itself. 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). 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.

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). That is a fixed limit locked in place by the combination of the computer’s BIOS and the drive’s ATA version. Back in 1986, 500 MB must have seemed like an incredible amount of storage space because the largest PC hard drives in those days were about 40 GB.

Some older PCs with an Award or AMI BIOS have a capacity limitation of 36 GB (33.8 GiB), and “traditional” ATA has the 137 GB/128 GiB limitation noted above, so it can be problematic adding drives over 128 GB to older PCs as well as most pre-2002 Macs.

And then there’s the operating system, which may have even lower capacity limitations than the drive itself and the PC’s BIOS support:

  • MS-DOS and Windows 95 (original version) top out at 8.4 GB.
  • Windows 95 (revision 2) and Windows 98 can use 128 GB/137 GiB.
  • Windows XP requires Service Pack 1 for drives over 128 GB.

More recently, as hard drives have passed 2 TB capacity, we’ve run into another capacity problem. Old BIOS-based PCs and older Macs have a 2 TB ceiling, and the workaround is a PC or Mac that uses EFI and runs a 64-bit operating system.

Mac Size Limitations

Early versions of the Classic Mac OS (prior to 7.5.2) have a 2 GB limit per partition, so higher capacity drives need only be partitioned into multiple 2 GB (or smaller) segments. For Macs running System 7.5.2 through 8.0, the limit is 4 GB (specifically 4,063 MB) per partition, and PCI-based Macs can access up to 2 TB partitions.

The Classic Mac OS can run into problems when run from a partition larger than 8 GB, and on early Macs that support Mac OS X, the boot partition not only needs to be smaller than 8 GB, it must also reside within the first 8 GB of space on the hard drive.

Prior to the Quicksilver Power Mac G4, introduced in Mid 2001, Macs had the 137 GB/128 GiB limitation noted above as detailed in How Big a Hard Drive Can I Put in My PowerPC Mac?. Although Apple doesn’t specify ATA versions in its technical specifications and Quicksilvers are widely reported as having Ultra ATA/66, yet with Mac OS X 10.2 and later, the Quicksilver 2002 supports big drives.

Mac OS X Maximum Drive Capacity

  • OS X 10.0-10.1.5, 2 TB maximum volume size
  • OS X 10.2-10.2.8, 8 TB
  • OS X 10.3-10.3.9, 16 TB
  • OS X 10.4 and later, around 8 exabytes (8 million terabytes!)

On to SATA

SATA supports 48-bit addressing, so there is no realistic limitation on drive size.

* Apple began moving Macs to ATA in 1994 yet continued to use SCSI hard drives on its pro-oriented machines until 1997, when the Beige Power Mac G3 became the first pro model to ship with ATA by default. And starting in 1999, no new Macs shipped with SCSI ports by default, although some Power Macs could be ordered with a SCSI PCI card. Mac OS X doesn’t even include SCSI drivers, although contemporary SCSI cards often have built-in firmware support so they can even function as boot drives, while older ones may have OS X drivers but are not bootable since they require those drivers to function within OS X..

Further Reading

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