Failed to open the disk image file /tmp/tde-ubuntu-14.04-desktop-latest-amd64.iso.

Could not get the storage format of the medium '/tmp/tde-ubuntu-14.04-desktop-latest-amd64.iso' (VERR_NOT_SUPPORTED).

Result Code: VBOX_E_IPRT_ERROR (0x80BB0005)

The problem is that the file size is not a multiple of 2048.

Padding it with

printf '%1536s' ' ' >> tde-ubuntu-14.04-desktop-latest-amd64.iso

makes VBox happier about that image.

///

 2048 - (.iso size mod 2048) = printf size