If your Toshiba external hard drive has been partitioned with the wrong file system, you will also encounter the "external hard drive not recognized" issue. For instance, you may have formatted the drive with the ext4 file system from Linux or the HFS Plus file system from a Mac. Or, you drive file system is damaged and shows as RAW.
However, Windows cannot recognize these file systems, thus Windows cannot recognize your external hard drive.
First, recover important files from the external hard drive that is not detected by Windows. Second, reformat this drive with the newer NTFS file system or older FAT32 file system so Windows will be able to recognize it.