How to fix mounting problem with external drive?
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: https://apple.stackexchange.com/a/29914/44080