Bitock key

how do you unlock bitlock when you have lost the key.

You don’t,. Wouldn’t be much good if you could…

So I can’t get back into the external hard drive without the bitlock key

Well, I’m not sure what you mean by “bitlock” key… That’s not a WD term. BitLocker is a Microsoft technology.

But either way, you’re not going to get to your data if your key is lost.

But you can always just wipe the drive and re-use it, if that’s what you’re trying to do.