My Toshiba external hard drive is not being detected on Windows PCs, but works flawlessly on my Mac.
File system is NTFS.
Assuming it's a USB disk you could start by downloading a USB utility like "usbdeview" to see whether the USB controller is recognising a device connected. If not, check your cabling.
First you need to ensure that your USB hard drive is being detected by the windows OS.
Open Device manager
View by device connection
You should be able to see multiple PCI-to-USB bridges
Search under that and see if you see the "USB Mass Storage Device"
For example, I see:
+ PCI Bus + ... + Intel (R) ICH9 Family USB2 Enhanced Host Controller - 293A + USB Root Hub + USB Mass Storage Device + Seagate FreeAgent Go USB Device
If you see it correctly, then the USB device is detected.
Then what you need to do is to see whether the filesystem is correctly detected.
Open Computer Management.
You should be able to see it mounted.
Tell us where you can't follow and let us help you =)