1. Resize VDI with File > Virtual Media Manager. If you are using snapshots, remember to resize all snapshots.

    Reference: resize vbox disk.vhd with snapshots | SpiceWorks.

  2. Resize with GParted Live CD.

    Reference: How to resize a Virtual Drive | VirtualBox Forums

  3. If you are using LVM, run:

    df -h # view volume path
    lvextend -l +100%FREE <VOLUME_PATH>
    resize2fs <VOLUME_PATH>

    The <VOLUME_PATH> should be something like /dev/VolGroup/lv_root.

    The usage of the 2 LVM commands can be found in this post.

    Reference: How to extend root partition on LVM in VirtualBox with snapshots | SuperUser

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