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November 9, 1998

Ultra DMA drives may have difficulties with Windows 95/98, NT

Progress marches on, and progress brings us new technologies. One technology that keeps changing is hard drives. As PCs have gotten faster, hard drive controller manufacturers have tried to keep up by supporting ever-newer standards.

  • In the old days, when the IBM PC/AT sported a 16-bit bus, the 16-bit ATA standard was developed for hard drive controllers. This standard is better known today as IDE, or integrated drive electronics.

  • A new standard, ATA-2, also known as Fast ATA or Enhanced IDE, introduced better Direct Memory Access (DMA) modes. These modes speed up disk reads and writes.

  • The latest standard is variously called Ultra ATA, ATA-33, DMA-33, Ultra DMA, or simply UDMA. UDMA theoretically can support a maximum "burst mode" transfer of 33.3MBps. This is an improvement vs. the 16.6MBps maximum rate for DMA transfers using the original IDE standard, although you won't actually hit these ideal speeds in real-life use. For more information on these standards, see

If your system's BIOS, chip set, and hard disk support UDMA, the installation of Windows 98 automatically loads driver support and enables bus-mastering DMA transfers. You can also install such drivers for Windows 95 and Windows NT, and manufacturers of such systems often do this for you.

I'm hearing of problems with some systems' reliably handling the higher-speed transfers, however. Mark Stapleton, a mechanical engineer at Georgia Tech Research Institute, has done extensive research and found that the higher speeds can cause a variety of difficulties in some cases.

  • Seagate Technology has prepared a fix for three early production versions of its Medalist Pro 7200 rpm hard drives. If a drive controller has been configured for UDMA support with these drives in Windows 95 and NT and then Windows 98 is installed, the Windows 98 installation crashes. This also occurs with some other manufacturers' drives, and Seagate is to be commended for publicly posting on its Web site the exact model numbers affected and offering a fix. See, or call Seagate technical support at (405) 936-1200.

  • Microsoft reports that a Windows 95 driver can cause PCs to crash while accessing a hard drive using UDMA if a hardware error is encountered. The driver may also read or write incorrect data when a hard drive is recovering from a "suspend" state. Both of these problems and others can be corrected with an updated driver, which is available at

  • By far the most frustrating problems, according to Stapleton, are intermittent crashes or data errors caused by UDMA transfers at speeds that are not quite reliable. He and others point to the old IDE cable that many PC manufacturers still use to connect drives to the motherboard. The cable is not shielded against electrical interference, which can be a problem at high transfer rates. Circuit Assembly sells a special ATAS cable for a price of $12.99 plus charges for shipping that is well grounded and may solve this kind of trouble. See

I'll add more next week on resolving other UDMA difficulties. My thanks to my co-author of Windows 98 Secrets, Davis Straub, for suggesting I investigate this topic.

Brian Livingston's latest book is Windows 98 Secrets (IDG Books). Send tips to He regrets that he cannot answer individual questions.

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