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If the WD drive was connected to the iMac during the upgrade then it may have gotten corrupted.
Try connecting it to a Mac running an older version of OS X, or another Mac running OS Yosemite, and let me know if any of them recognize it. If it is not recognized by a Mac running an older version of OS X, then it is likely that it has been damaged.
These are the only tests that you can do. It will help to confirm if the data is recoverable, and if it is then you can try copying the data onto another Mac, reformatting the HD, and transferring the data back onto it again to see if that works.
To answer you question again, you cannot mount this external HD to your Mac - you must either try using another Mac, or send it away to a data recovery service.
Let me know if you need any further assistance with this question.