For the best possible performance, hard disk drives should be set to use Ultra DMA mode. This allows the disk controller...
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to transfer data from the drive directly into system memory (fast) without requiring the intervention of the CPU (slow).
All hard disk drives produced today support some form of Ultra DMA. They also support the older method of data transfer, Programmed I/O (PIO), which uses the CPU to handle the data transfer and is correspondingly slower. By default, Windows performs the fallback when it encounters six cumulative timeout errors.
In other words, after multiple consecutive read errors, hard drives may fall back from DMA mode to the slower and more CPU-intensive PIO mode. After doing some research, I've found that CD and DVD drives are also affected by the problem. But in their case, the problem may not be as readily noticed, and requires some specific intervention.
I was looking in a drawer and found a DVD-R disc that held a lot of data I wanted to dig through. However, the disc was badly scratched, and when I tried to read it from my DVD drive, the computer struggled with it for minutes on end and ended up not being able to mount it at all. (In fact, the drive locked up and refused to eject the disc, which prompted a reboot.) Soon after, I found the data in another form and decided to re-burn it to another disc, only to find that my DVD burner (the same drive I'd tried to read the damaged disc from) was running very slowly.
On a whim, I looked at the entry for the drive's ATAPI controller in Device Manager and found that that the channel the drive was registered on was running in PIO mode. Evidently the number of consecutive read errors from the drive had caused it to fall back to PIO mode and stay there. My fix for the problem was to uninstall the IDE channel from the Device Manager and reboot. This forced the computer to redetect both the IDE channel and its associated devices, and reset their error counters.
In my previous discussion of the problem, I pointed readers to a Microsoft hotfix (since implemented in Windows XP Service Pack 2) that addresses the problem. The fix can also work in conjunction with a Windows Registry setting which will only perform the fallback after six consecutive errors, rather than six cumulative ones. This may help if you're dealing with a number of CDs or DVDs that are difficult to read, prefer a less aggressive fallback policy, and don't want to have to force a reinstallation of the controller quite so often.
Note: This fallback does not happen with IDE/ATAPI devices that are attached through a bridge, such as a USB or IEEE 1394/Firewire adapter.
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About the author: Serdar Yegulalp is editor of the Windows Insight, (formerly the Windows Power Users Newsletter), a blog site devoted to hints, tips, tricks and news for users and administrators of Windows NT, Windows 2000, Windows XP, Windows Server 2003 and Vista. He has more than 12 years of Windows experience under his belt, and contributes regularly to SearchWinComputing.com and SearchSQLServer.com.