You can attach new storage to your Virtualbox virtual machine (VM) after it's already been created. This storage can either be from virtual hard disks of supported format commonly used by other virtualization platforms such as VMware or QEMU or create a new one.
The new storage can be attached to either the IDE or SATA controller, depending on your preference and requirement.
Steps to add new hard disk to VirtualBox VM:
Launch
VirtualBox.

Make sure the
VM is powered off.

Right click on the
VM from the list and click on
Settings.

Go to
Storage tab.

Select which controller you want to add the disk to and click on the
Add new storage attachment button.

Select
Hard Disk from the list.

Click
Add to add existing disk.

Select from existing disk and click
Open.

Supported format includes VMDK (.vmdk), VDI (.vdi), VHD (.vhd), Parallels (.hdd), QED (.qed), QCOW (.qcow, .qcow2) and VHDX (.vhdx)
Click
Create to create new hard disk.

Choose type and click
Continue.

Choose disk allocation type and click
Continue.

Choose the size of hard disk, select location and filename (optional) and click
Create.

Select added or created hard disk from
Not Attached section and click
Choose.

Click
OK.

Launch the
VM to configure the newly added hard drive on the operating system level.

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