As we know, if you connect a new Toshiba external hard drive to your PC, your Windows may not recognize it because it has not been initialized. Open Disk Management. (Press Windows + R on your keyboard to launch the Run dialog box. Type "diskmgmt.msc" into the box and press OK.)
Right click on the name portion of the disk entry, where it says Disk [#]. Select Initialise Disk from the right-click context menu. And then, you will be given detailed operating prompts. Just do as told.
Note: during the initialization process, you'll be prompted to choose whether you want to use a Master Boot Record (MBR) or a GUID Partition Table (GPT) for the partition style of your Toshiba external hard drive. Here, in my opinion, if the capacity of your Toshiba HDD is over 2TB, you had better choose GPT, for MBR can support up to 2TB disk.