Please do the trouble shooting as following:
1. If your drive has been used under Windows, it might be in Windows NTFS / FAT format, you will need to change the partition format to GUID partition or Apple Partition Map in order to format and mount the drive in disk utility.
For more information about partition in Mac OS disk utility, please check: How do I partition my drive in Mac OS X®?
2. There is a chance that some background process may lock the drive and prevent it from being formatted, please turn off Spot Light and Time Machine.
For more information about Spot Light and Time Machine, please visit:
Mac 101: Time Machine
Why my VR or VR mini is slowing down?
3. You may want to login to Terminal mode and check if there is any process locks down the drive for example: fseventsd process.
Fseventsd is a file system “events” notifier, running as a daemon process, used by Spotlight, Time Machine and other 3rd party softwares. It is to log the changes in the file system. Events are logged in the hidden “.fseventsd” directory at the root of the drive.
To terminate the Fseventsd process, login to Terminal and type in:
“lsof | grep” then drag the drive to the terminal window to complete the path to the drive’s mount point, and press enter. You may see Fseventsd process and it’s process id in the list, use “sudo kill -9” to kill this process.
Additional Information: List Open Files (lsof) is a useful tool that will show you opened files. In Mac OSX, Unix and Linux everything is a file, so you can use this command to monitor opened processes, files, devices…etc.