The first thing you need to do is determine where the problem is. There are 4 possible choices:
The actual My Book Thunderbolt
The cable between the My Book Thunderbolt and your Mac
The power adapter from the My Book Thunderbolt to the wall
Based on the symptoms, it could be any of them although it’s less likely to be the power adapter than the others.
To determine where the problem lies, the first thing to do is to try a different data cable to see if the drive is recognized. If it is not, than try the drive (with both cables ((one at a time)) on a different computer. Is it recognized there?
If it is not recognized on multiple computers with multiple data cables the problem is either the power adapter or the device - as you just purchased it, I would take it back to the store and exchange it for another one. Drives get damaged pre-purchase all the time - for all you know it fell off the shelf in the store.
(replacing <Volume Name> with the actual name of the volume)
I can’t see why a hidden flag would have been set, but just want to rule it out as a possibility…there’s a way to do this in the UI, but I don’t know El Capitan very well and am not sure where its been moved to.
(My Book Thunderbolt 6TB drive)
I have the exact same problem as this and have followed all the steps with no improvement. I have bought a new cable which has no effect. I have tried a second mac. Drive shows in Disk utility and terminal but won’t mount. Full drive check using WD Drive utilities completes with no problems. Any ideas as I have a lot of data on the drive.