I have an external hard-disk that doesn't have a USB port. It works with my MacBook. Now, I'd like to connect it to a PC machine, so I'd need to know how to convert the cable to USB. I'm not sure the name of my Apple cable (but it's definitely not a USB cable) and I don't have any idea whether the conversion to USB is even possible.
The cable you have there is a Thunderbolt Cable (herein referred to as TB). You can get a TB to USB adapter, but you cannot get a USB to TB adapter.
First an understanding of Thunderbolt...
In short, TB is a combination of of the PCIe Bus and Display Port (herein referred to as DP). Thunderbolt Overview
Your PCIe Bus, simply put, is the interface that adapter cards use to connect different peripherals to your computer. While both PC and Macs have PCIe Interfaces, it's much easier to show the ones on a PC, due to their expandability. Rest assured, while these connectors aren't in your Mac, the bus is. It's how your HDD, WiFi, etc all interface with the CPU and each other.
Using the PC as an example again, suppose you want to add a network adapter to your machine. You just buy an adapter like the one below.
What TB does is take all that tech, put it on a cable and allow you to attach the adapter OUTSIDE your machine. So, for the example above, TB puts it all in a nice, neat package like what's below:
Or a TB to USB...
USB, for all intents and purposes, is just a serial data cable, it just has two data channels, a ground, and 5v power. It does not cary any PCIe bus traffic whatsoever - it just doesn't have the capability.
It's sort of like recording your favorite TV show on a DVR. While you get the finished product, there is no way for you to recreate all of the tracks, video frames, etc to turn it back into raw footage; it's a one way street.
Now...How do you connect that drive so that your PC can use it. Assuming you have desktop PC with an open PCIe slot, you can use something like this HP Thunderbolt Adapter
If it's a PC laptop, you won't be able to connect the drive, and your best best would be to share it across the network.
How do you do that?
It's fairly easy. OS X Daily has a very good write up already.
After you have gotten access to your files, I would look at switching out your drive for a USB model so that it would be compatible with all your computers.
That's called Thunderbolt. Due to the nature of Thunderbolt, you can't directly convert it to USB. You may be better off accessing the disk some other way, such as sharing it with your PC over IP.