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I have a USB external hard drive with AC adapter that works fine with an Ubuntu machine. But when I plug it into my MacBook Air it lists one partition on the finder, and does not mount the other.

Even though it is listed on the finder, it shows that there are no files on the drive (even though there are).

Tried both USB ports with the same result.

Disk Utility does not see it.

How to fix?

OSX 10.7.5

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Please edit your question to include what file system format (exFAT, HFS, etc) of your external HDD. –  Global nomad Mar 6 '13 at 0:21
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2 Answers

The answer to your questions is in the "Disk Utilities" application: just type this in the search box on the top-right corner of your screen, and launch the application.

For every device connected, you will see much information. Relevant to analyse your problem is the "format" information: HDFS, Mac OS Extended, ExFAT, FAT32, ext3, ext4...

I guess because your harddisk comes from Ubuntu, it must be ext3 or ext4. If that is the case, please read the following answer for further help: http://apple.stackexchange.com/a/29914/44080

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You have a partition that Finder can display but does not show in the disk utility? If this is true, that is a major bug that you should report to Apple using bugreporter.apple.com –  hadinbe Mar 8 '13 at 16:06
    
I'd guess you are correct that one of the volumes is a format for which OS X doesn't automatically mount. I'll add a second answer rather than editing yours to disclose how to dump the current filesystems that OS X supports... –  bmike Feb 10 at 13:26
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You can determine exactly how OS X is seeing the drive with the diskutil command.

diskutil list

The command above will list all discovered filesystems. Once you see the number the kernel assigned your specific disk (probably disk1 if it's the first drive connected other than the bootable drive, disk0) then you can get more information for each diskX and diskXsY with X and Y being the digits assigned to each whole disk and each partition on the affected physical or logical volumes:

diskutil info disk1
diskutil info disk1s1
diskutil info disk1s2

Once you have this level of detail, you may discover that the partition scheme or some other aspect of some of the volumes needs adjusting so that it will mount on OS X as well as on Linux. You can see which filesystems your Mac has support for with the command diskutil listFilesystems. Once you've discovered the filesystems in play, you can either add a driver to OS X for your chosen scheme or repartition and reload the data from Ubuntu if you need that partition to mount on OS X as well as Linux.

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