How to Enlarge Disk Image in VirtualBox?
Your vdi path is wrong:


  VBoxManage: error: Could not find file for the medium


In your resize command either use the uuid of the vdi or a proper path (no backslashes!) :

VBoxManage modifyhd "/Users/masi/VirtualBox VMs/Ubuntu 14.04 64bit/Ubuntu 14.04 64bit.vdi" --resize 10000


or (with backslashes but no quotation marks)

VBoxManage modifyhd /Users/masi/VirtualBox\ VMs/Ubuntu\ 14.04\ 64bit/Ubuntu\ 14.04\ 64bit.vdi --resize 10000


or after entering VBoxManage list hdds:

#replace UUID and size below by the appropriate UUID and the required size
VBoxManage modifyhd UUID --resize size




If your PATH doesn't work properly you may have to prepend /usr/local/bin/