Is there other stuff on the disk you need.. if not simply use the airport utility and erase the disk.. a quick erase will take less than 2min..
If there is stuff you need on the disk you need to use terminal command to delete the file.. but I am unsure what you mean by 400gb disk .. is it a sparsebundle from Time Machine or a dmg file or what is it exactly.
Did you try the instructions from Pondini.
Q12 here. http://pondini.org/TM/12.html
It does not cover Mavericks.. which is strange and has some issues with permissions rather worse than previous OS.
You may find manually mounting the TC helps.. particularly just having the disk password set correctly.
A TM backup is easier to delete from the computer that created it.. Is that available??