To Resize Fixed Disk Must First Make Dynamic

[ehowton]

VBoxManage modifymedium disk ef0b9dad-02c9-4800-924a-ae96ef7f9c27 --resize 16384
Progress state: VBOX_E_NOT_SUPPORTED
VBoxManage: error: Failed to resize medium
VBoxManage: error: Resizing to new size 17179869184 is not yet supported for medium 'Win7_fixed.vdi'
VBoxManage: error: Details: code VBOX_E_NOT_SUPPORTED (0x80bb0009), component MediumWrap, interface IMedium
VBoxManage: error: Context: "RTEXITCODE handleModifyMedium(HandlerArg*)" at line 768 of file VBoxManageDisk.cpp

Must also be a VDI, so may need to include "--format VDI" before -variant:

vboxmanage list hdds
UUID: ef0b9dad-02c9-4800-924a-ae96ef7f9c27
Location: /VMs/Win7_fixed.vdi
Storage format: VDI
Capacity: 10240 MBytes

VBoxManage showmediuminfo ef0b9dad-02c9-4800-924a-ae96ef7f9c27
Location: /VMs/Win7_fixed.vdi
Storage format: VDI
Format variant: fixed default

VBoxManage clonehd Win7_fixed.vdi Win7_dynamic.vdi -variant Standard (Standard denotes dynamic)
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

vboxmanage list hdds
UUID: ef0b9dad-02c9-4800-924a-ae96ef7f9c27
Location: /VMs/Win7_fixed.vdi
Storage format: VDI
Capacity: 10240 MBytes

UUID: a57b7d9a-0cee-40fc-95a8-02797611efd6
Location: /VMs/Win7_dynamic.vdi
Storage format: VDI
Capacity: 10240 MBytes

VBoxManage showmediuminfo a57b7d9a-0cee-40fc-95a8-02797611efd6
Location: /VMs/Win7_dynamic.vdi
Storage format: VDI
Format variant: dynamic default

VBoxManage modifymedium disk a57b7d9a-0cee-40fc-95a8-02797611efd6 --resize 16384
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

Then you can resize the partiion: https://www.howtogeek.com/124622/how-to-enlarge-a-virtual-machines-disk-in-virtualbox-or-vmware/