Q:

Can't mount or repair Transcend 1TB external HD

I have a Transcend 1TB external hard drive. The hard drive shows up in Disk Utility but when I click on Mount, the LED of the drive blinks for 2 seconds.  And then nothing happens.  When I clicked on Repair I got this message:
Repairing file system.
Invalid content in journal
Checking Journaled HFS Plus volume.
Detected a case-sensitive volume.
Checking extents overflow file.
Checking catalog file.
Invalid node structure
The volume HD1 could not be verified completely.
Invalid content in journal
File system check exit code is 8.
Updating boot support partitions for the volume as required.

I don't think that's the problem but my hard drive was almost full and I clicked on backup. And I think the problem happen at this moment.
I tried another USB cable, and on Windows but without success.
Any idea that involves not losing my data is very welcome :)

OS : El Capitan 10.11.6
MacBook Pro (late 2008)

A:

Try using a great app for Mac like Disk Drill, Data Rescue 4 or R Studio to recover your own data, another option is to bring it to a computer store, and they can do it for a fee, as Best Buy does for around $450-$1200. If you prefer a free way, then I recommend using TestDisk, an open source project that will help recover and perform sector by sector scan drives. 
I would just leave the drive plugged in and use one of the applications, if the drive is still able to spin (as I'm assuming it is because it will populate in disk util). In Disk Drill, you can click the settings icon beside your drive and click "deep scan". For a 4TB HDD, it takes around 3 days. It should be fine if left running for a few days, and eventually, it will allow you to mount your old drive, and transfer the files to a new drive. Docs for TestDisk are available here
Best of Luck! 

