Hands on: How to upgrade your hard disk

The 137GB limit

For a PC to recognise IDE hard disks bigger than 137GB, its Bios and operating system must support 48bit Logical Block Addressing (LBA). The original LBA specification was introduced to solve another capacity issue, except back then it was to support disks larger than 504MB.

Following the introduction of LBA, a compatible Bios could recognise disks bigger than 504MB and compatible operating systems could format and address the additional space.

As with so many developments in the IT world, though, what seemed to be an absurdly large upper limit at the time was reached surprisingly quickly.

LBA employed a 28bit addressing system with an upper limit of 137GB, more than 250 times greater than the previous ceiling it was developed to break through. But before long, hard disks approached this 137GB limit and the technology needed updating.

Enter 48bit LBA, which extended the upper limit to what seems like an astronomical 144,000,000GB, or 144petabytes. No doubt this will seem restrictive in a few years, but today it allows plenty of room for growth.

So in order to use the full capacity of a disk larger than 137GB, you’ll need a Bios and operating system that support 48bit LBA. If your PC is more than a couple of years old, it’s best to check the Bios update pages of your motherboard manufacturer’s website.

The updates should state, among other things, support for certain sizes of disk. If your system isn’t too old, there might be a Bios update available that supports 48bit LBA.

If so, follow your motherboard manufacturer’s instructions to download the file and flash the Bios. If a suitable update is not available, you’ll either need a new disk controller or an external solution, which we’ll discuss later.

In terms of operating system support, only the latest versions of Windows come with 48bit LBA support built in and enabled as standard. Owners of Windows XP will be fine so long as they’re running Service Pack 1 or greater.

Owners of Windows 2000 will need Service Pack 3, but will also need to perform a small Registry hack. This is detailed in Microsoft Knowledgebase article 305098).
Things get trickier for owners of Windows ME and 98. Neither has native built-in support for disks larger than 137GB, although third-party controller cards or external drives can provide Bios support for 48bit LBA, and normally come with drivers that allow the OS to exploit them.

Windows 98 owners won’t necessarily be completely in the clear, as the version of Scandisk that comes with it doesn’t support partitions larger than 137GB.

Fdisk may present problems, mistakenly identifying the capacity of a large disk – to be safe, partition the disk using a percentage rather than entering an amount in GB, or partition and format the disk using a more recent operating system.

If you do partition and format the disk on another machine for use on a Windows 98 or ME system, use the Fat32 file system, as neither of those operating systems can recognise the NTFS file system.

Since support for a large disk on an older system will almost certainly be provided by a third-party controller, it’s best to consult its documentation on how best to install the drive. And unless you’re absolutely confident of sufficient support for disks larger than 137GB, play safe and install smaller disks such as a 120GB model.

As for Windows 95, the OS itself doesn’t natively support disks larger than 137GB, and you’re unlikely to find a third-party disk controller that will do the trick. Again, best to stick with 120GB disks or smaller.