virtualbox.org

hi.
i need more space on debian.
IDE Primary Master Debian.vdi (debian installed on it) and
IDE Primary Slave a.vdi (not even formated)
how to mount a.vdi disk to debian and use it as 2nd hdd

Use a partitioning program like Parted (or GParted for a GUI) and create a partition. After that, format it so it get's a filesystem and UUID. Enter that UUID in /etc/fstab with the corresponding options and mountpoint and you're done. You can't mount a raw hard disk in Linux (nor in any other OS).