It can happen in Windows 10 at external drives that the paths or drive name are not correct assigned. But now, do not panic, here is the solution!
Start the Windows 10 Disk Management and verify the drive letter assignment!
Launch the Disk Management from the Windows 10 run dialog and command diskmgmt.msc and now you can, if required, the assigning unassigned drive letter for each partition. (... see Image-1) or format a external drive (HDD, SSD, USB etc.)
Info: A dynamic disk may become Offline on Windows-10 if it is intermittently unavailable or corrupted.
If the new disks appear as not initialized on Windows-10. Before you can use a disk, you must initialize it. If you start Windows-10 Disk Management after adding a disk, the Initialize Disk Wizard appears so you can initialize the disk.
FAQ 93: Updated on: July 12th 2015 21:19