Im guessing theres still some boot files on there, this happened to me.
Is that drive you want to format being classed as the system drive? Check for hidden files on the drive that are still lurking around an then use something like FDISK to format it outside of windows.
IIRC You can't copy the boot files over so it would mean a reformat, but thats one way of getting your nuisance drive formatted
Go check, whats the drive being listed as under drive manager, & What hidden files are left on there
