I see your need to get access to your files.
But if the drive is not mounting there is nothing we can help you with in the InDesign forum.
You can try to do something with the help of the «DiskUtility» which you find int the «utility» subdirectory of the «application» directory. Perhaps you can see the drive in there and fix the problem. Also have a look at the «Console» (in the same directory) when you plug the USB-drive into your MBP. There might be some hints why the drive is not mounting.
I wish you good luck.
Daniel (from Switzerland)
(If it's the only copy IT ISN'T A BACKUP. This is a painful experience, but I mention this because I often see reports from people who feel that "putting it on an external drive" is "backing it up" even if it isn't a backup of any original. The principle of backups is to have TWO DIFFERENT COPIES. For proper safety they need to be in different buildings, and have multiple backups).
Anyway, good advice from Daniell. If it still doesn't work. Try on a different Mac. If it still doesn't mount, it has probably gone faulty. There are companies which try to recover disks, sometimes fixing the circuitry, at considerable cost.
How old is your hard drive? Could you let us know about its history, any wear & tear on the drive? You need to check if your Seagate drive is producing grinding/clicking sound. Consider it to be physically damaged if it is producing erratic sound. The second kind of trouble could be file system of the drive. You can download Stellar software or testdisk software and connect your external hard drive for recovery. Try your luck with the data recovery app and let us know further progress.
It isn't in disk utility. Tried connecting to other Mac but it unmounted.
@mark I will download and try it on my drive. Thanks