MyBook not visible for DOS operations

I’ve owned 4 external hard drives - 3 WD and one from Toshiba.

Two of them, including a recently purchased MyBook, are invisible at start up - meaning, if I start from a boot disk they don’t show up and I can’t use them for bootable operations.    But once Windows XP is running they all work perfectly.

I still use an old version of Norton Ghost (2003) which copies from PC to external in DOS - this works perfectly with the visible drives, but fails with the invisible ones.

I don’t get it - I can’t see any difference between my two newest drives (apart from their sizes), I set them up the same way - yet one is visible and the other not.

But then I don’t know enough about what makes a drive bootable to be able to tell.

Is there a parameter or setting that I’m missing?    I’m sure this is an easy question for anyone with proper knowledge on the subject.

Many thanks ahead