I use VirtualBox 4.2.6 with Ubuntu 12.04 as a host and Win XP as a guest. I assigned a virtual disk (VDI) that grows with XP to a a maximum of 40 GB. Currently it uses 25 GB.
Within XP it repeatedly tells me that there is not enough space available.
I still have plenty of space on my host's primary disk
df -h /dev/sda1 143G 48G 89G 35% /
What to do to expand my XP's disk? This question did not help me, btw.