I too am having the same issue. Have tried all of the suggestions above but with no luck. Have a fair bit of music on there that I want to transfer over to another drive. Stopped mounting just after I put the files on there and wanted to back them up elsewhere for a second copy as they were recovered from a damaged drive. Drive worked fine for a while but then nothing. I can see the drive in disk utility but it won't mount at all. Have also tried other drives and the same is happening. Thought it might improve after the latest update to 11.3, but I guess not.