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DBTechnique

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MAC OS X: Problem with External disk seen with Disk Utility, but not mounted in finder

I usually use an external disk to boot MAC OS X with a 15" Mac book pro.
Now it does not work, when I try to boot as usual with Alt pressed when the computer starts, it provides the message: "You nees to restart your computer. Hold down the Power button for several seconds or press the Restart button".
When I boot with another method (the original internal HD), and I go through the Finder, I cannot see the external disk. But when I run the Disk Utility I can see the disk. I could repair it. but the volume is not mounted, and even if I try to Mount it, it tells me to repair it. But when I repair it, it tells me that the volume is fine.
I need to be able to reboot on this external disk, or at least to be able to import all data from it (that I cannot do now because I cannot see the external disk from Finder). Please advise.
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roylong
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Is the disk making any weird noises indicating a problem with the hardware?

You could try bringing up the system with the disk connected, drop out to the command line (Terminal application) and run the fsck -f (force) /device/disk

If you need more instructions on fsck let me know.
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When you boot, have you tried holding down the Option key?  This will show you the boot options available.
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DBTechnique

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The External HD is a SSD that works fine for more than 1 year. It is connected with the MAC BOOK PRO through the USB (We tried all the USB ports). Anyhow, the disk is detected fine in Disk Utility, only the Volume is in gray despite of the fact that size and name is correct and the Repair, or Verify work fine from the menu First Aid.
Pressing the key Option is the only way we can select the External HD to boot with it, so indeed, we did it.
But since when I boot on the main HD, the External HD is not mounted, we can guess that indeed, we cannot boot with it.

Can you tell more about the command fsck -f (force) /device/disk
How does it work? How to be specific about the HD to target?
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roylong
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Do you get this error message when you try to boot Mac without this drive connected?
Does the disk get its power from the Mac's USB port, or does it have its own power supply connected to the 110 AC? If it is getting its power from the Mac, try hooking it up to a powered USB hub instead.
Also, does it not mount after you repair it using Disk Utility? Have you tried mounting it using the MOUNT button in the Disk Utility tool bar?