you are saying I should copy everything from my passport, format it in windows 8.1, copy back, then switch to 10?
How full is the drive?
if plenty of space, you could partition the drive and move your important files to the new partition
http://wdc.custhelp.com/app/answers/detail/a_id/3865/related/1/session/L2F2LzEvdGltZS8xNDQwMzg0NDU0L3NpZC9qZUJPNkh1bQ%3D%3D#win8