How to run windows 8 from external hard drive?

Hello everyone,
A few months ago my laptop broke and it needed repairs. The HDD was replaced and i got my laptop back with one new HDD inside and one "broke" HDD in a bag. Later the "Repaired" Laptop crashed again so i bought a new one and took the HDD out of the old laptop. I bought a HDD case with USB connection and both of the HDD's where intact. The oldest HDD contained windows 7 and the replaced contained windows 8. Now i want to sell the Windows 8 HDD so i want it to be clean of personal files but i want it to keep windows 8. I thought the solution was to run the windows 8 on the HDD so i can use the program deletation program of that windows 8. The problem is i dont know how to run windows 8 of a hard drive. I have searched the internet but with no succes. I know its possible but i dont know so i ask you guys if you can help me.
Greetings Sebas

Why not just wipe the drive so it's blank, delete all the partitions on it. There is no point in trying to keep the Windows 8 install that is on it. You can't sell a preinstalled OS like that, not legally anyway. It's going to fail activation anyway when put in another PC that has different hardware in it than the one it was originally installed on.
Windows To Go will run from a USB drive but you have to have Windows 8 Enterprise to create the drive. Your normal Windows 8 install isn't meant to run like that.

Okay Thanks,
Your advice is just to delete everything of the drive including windows 8 because it will not work on an other PC because of hardware, drivers and maybe it wont even open because the new computer has an other IP adress. Thats pretty clever.
I beleve i just have to format the disk right? or something else?

Yes, Windows isn't meant to be transferred from PC to PC like that. Doing it usually just opens a big can of worms and gives those involved a big headache.
If the disk only has one partition on it formatting it will erase everything. If it has more than one partition on it you'll have to format each one. The quick and easy way is to just go to disk management and delete all the partitions on the disk, and then create one new one. If you have sensitive info on that drive you may want to go one step further and do a clean all.
Disk - Clean and Clean All with Diskpart Command - Windows 7 Help Forums

Partition or Volume - Delete - Windows 7 Help Forums