You should see at least two, sometimes three, entries for your drive depending on how it is formatted. If you are running 10.12 or 10.11 this step is not required as the option is not available. Change the view to Show All Devices instead of the default which is Show Only Volumes. If you are running macOS 10.13 (High Sierra) or later it is important to change the view in Disk Utility.
Note: Reformatting the drive will erase all data on the drive, so you should copy any data you want off the drive prior to formatting. Not sure what version you are using? Click here to determine your version of macOS. This article applies to the following versions of macOS:
The following is not an exhaustive list of error messages, so you may not find your specific error here. This article covers some of the common error messages and questions you might encounter while using Disk Utility.
