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I have a USB drive that I use with my Mac for a Time Machine backup and I will sometimes unplug it without ejecting it. Whenever I do, Time Machine isn't using it and, as far as I know, nothing else is either. (Spotlight isn't indexing either when I unplug it.) Is it a bad idea to do this? Will this do any damage to the drive, even though nothing is using it?

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Potentially yes.

To prevent storage from wearing itself out from all of the minuscule writes a system may request from a disk, those writes are usually buffered and then flushed all in one go. When you manually eject a disk, you flush the write buffer, and in doing so, ensure that all of the data that's supposed to be on the disk actually is.

If you unplug a disk without "ejecting," i.e. flushing the write buffer, you open yourself up to the possibility of filesystem corruption or lost data.

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At least on Windows, write caching is disabled by default for USB devices. I believe this is the same on OS X. –  jcrawfordor Aug 25 '11 at 2:11
    
@jcrawford The software developer who though of that should get a cookie! –  NReilingh Aug 25 '11 at 3:06
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@jcrawfordor according to superuser.com/questions/113019/… that's not true... –  dertoni Aug 25 '11 at 7:18
    
You're right. Strange that OS X doesn't default to this clever setting. –  jcrawfordor Aug 25 '11 at 15:13

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