External Drive - Laptop

Of course you can use it as an external drive. But it really depends what you want to use the external drive for. If it is because you want to boot your m$ OS that has been installed to that drive from the external drive, then no, you can't use it for that, or not easily.

For Windows 7 you would need to do a special installation to that drive, via a virtual machine, for Windows 8 you can do the same, but with the Ultimate edition you also have the option to use "Windows to Go", which is easier to setup. Either way, having USB 3 would be recommended as otherwise the speed can be an issue.

For Linux on the other hand there is absolutely no problem installing the OS to the external drive and booting the PC from it. Here even using USB2 gives you acceptable speeds for most circumstances.

If it is just a drive for your Data or your backups, then using it as an external disk is absolutely no problem.