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I had a client recently who thought I'd be copying data and try read of the same drive at the same time - and according to him it's something you can't do because you'd damage the data/hard-drive.

I still couldn’t find any information as to why you can’t read and write on the same drive ‘at the same time’; I thought this is what hard drives are meant to do: writing and reading requests are split into sub-processes, which are then executed, not 'simultaneously', but one by one. You usually try to avoid reading and writing on the same drive ‘simultaneously’ - because this back and forth, and sharing only one connection, slows down both processes in the end. But that’s for speed considerations, not data or hardware safety - I thought.

It is ‘safer’ to first finish a copy as quickly as possible, before doing anything else - just because the chances of a power failure, anyone spilling a coffee or bumping your hard drive etc are obviously slimmer in a shorter period of time than in a longer one. But otherwise:

Can reading of a drive while copying onto it 'damage' the data you are writing, or the hard-drive itself?

If yes: How?

  • As the answers have said, there will be absolutely no damage. However, multiple access, whether reads, writes or a combination thereof will slow access times slightly as the drive heads will continually have to reposition. – Mawg Feb 10 '15 at 10:13
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Your analysis and @Tibors answer is correct, simultanously reading and writing a drive will not damage it - they are designed for this purpose.

If you think about it, it makes sense - There is no mechanism in the drive that would create a risk - you have read heads and write heads and an arm which moves across a spinning platter - and you additionally have logic to read and write this. Nothing about this process is likely to cause premature drive failure - as there is only 1 actuator motor it can't get 2 signals and jam up.

I can't even think how this myth may have been started - of-course its true that if a drive is failing you should avoid doing anything save reading your data off it onto a new drive - but thats not really relevant to the question. (I wonder if it might have something to do with ancient Magnetic-Core drives which were, of-course a totally different beast predating hard drives)

  • just fyi, there are not "read heads" and "write heads". The same head is used for both functions. Different electronics are used for the two functions but for each surface there is just one coil, one set of pole pieces, one head gap, etc. – Jamie Hanrahan Apr 2 '17 at 12:33
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Simultaneous reading and writing will not damage the data or the drive under normal conditions. E.g., this is what database servers continuously do for years. It does cause more seeks, but it's not enough to really affect drive lifetime.

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