DMA question

Since I haven't found the answer to this point by 'Googling' I am even more convinced it's a dumb question. Apologies in advance if so ....

Can external hard drives (Firewire or USB connected) make use of DMA in the same (or a different) way to internal IDE drives? I assume not, but ...?

The reason I ask is that I have a 500GB IDE drive which is faulty, in that as an internal drive it always switches back to PIO on reboot (I troubleshooted/ troubleshot (?? both sound wrong!) this for weeks - the drive is faulty, 99.99% sure: no way could I get it to stay in DMA mode after weeks of online expert help). I got a replacement drive, which works fine, so I am not interested in troubleshooting the problem longer, but the seller didn't want the faulty drive back, so I am wondering if it will be OK to put in an external caddy and use it as backup storage via a USB 2.0 cable. Would the fact that it has this DMA problem as an internal drive make any difference to its performance as an external drive (one I would hardly ever need to access apart from emergency restores). Or would it somehow increase the risk of data loss and CPU problems?


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  1. Since you have a defective HDD, I wouldn't put too much faith in its longevity. That being said, you should be able to use as an external HDD (USB enclosure) without any trouble. Because of the IDE to USB "conversion" in the enclosure, operating in PIO or DMA mode will be a moot point. I don't see any risk of increased data loss or CPU problems with this config, beyond the fact of using a faulty HDD that could die anyway.

    BTW, I think the answer to your "Can external hard drives (Firewire or USB connected) make use of DMA in the same (or a different) way to internal IDE drives? I assume not, but ...? " question is NO. The delivery of data is via the USB or FW source. The "translation" happens within the adapter that the HDD plugs in to inside the external enclosure.

    Good luck!
  2. Well I put it into a caddy and it seems tpo be working OK. Time will tell!

  3. "DMA" refers to "Direct Memory Access", and it's the ability of the I/O adapter to transfer data to or from memory directly without the CPU itself having to perform I/O instructions for every byte.

    External drives connected via USB DO operate in DMA mode (as do lots of other devices such as Network Adapters), but the USB 2.0 protocol specifically is much slower than an IDE connection and maxes out at around 30-35MByte/sec in most systems.

    So while a USB connection in DMA mode would use up a lot less CPU time than an IDE connection operating in PIO mode, the USB connection might still be a lot slower in terms of transfer rate.
  4. OK, thanks for that. It won't matter too much if so (although I didn't notice in practice that transfers were slow - though I only did one test transfer so far), as I will use the drive as a second backup for vital files and folders (first backup is one something I know to be reliable - famous last words!) and once files (like drive images and so forth) are on it I shouldn't need to access them much if at all.

  5. I wouldn't trust it.
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