You can determine exactly how OS X is seeing the drive with the
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.