[Teknikal]

If it was me I'd just use 7zip to archive the entire library, it should keep the structure when you unzip it where you want on a new drive. I keep a backup simply by right clicking on my calibre library folder and adding to a .7z file although I'm sure you could use any zip or rar program to do the same.

If you really just want the epubs/mobis alone though you could always just search *.epub or *.mobi on your pc, hit ctrl+a to select all, then ctrl+c to copy, finally just use ctrl+v to paste them to any folder you wanted.

edit: HarryT's post above mine would work fine as well just copy the entire folder untouched.