My host OS is OS X El Capitan. My guest OS, OpenBSD 6.1, ran out of disk space, so I ran
VBoxManage modifymedium disk OpenBSD\ 6.1.vdi --resize 15000 to resize my VDI from 4GB to 15GB. However my guest OS isn't using the additional space to expand:
[me@puffy:~]$df -lh Filesystem Size Used Avail Capacity Mounted on /dev/wd0a 907M 378M 484M 44% / /dev/wd0e 491M 467M 36.0K 100% /home /dev/wd0d 2.7G 2.6G -6.7M 100% /usr
I tried forcing it to grow by running
[me@puffy:~]$doas -uroot sh -c 'yes > /yesfile' /: write failed, file system is full
but it ran out of disk space almost immediately.