SDB:Create 10 GB file instantly with "dd"
To find out how much is free you can use from command prompt:
The output will look like this:
Filesystem 1K-blocks Used Available Use% Mounted on /dev/hda2 40321836 28054876 12266960 70% / tmpfs 257892 0 257892 0% /dev/shm
This means that you have some 12 GB of unused space, so you can proceed with next command that will create 10 GB file:
dd if=/dev/zero of=myharddisk.img bs=1000 count=0 seek=$[1000*1000*10]
This trick will work on ext3, ext4 & reiserfs - but will NOT work on FAT32 partitions as they have maximum file size of 4 GiB.
Also because this file is null from within - some programs will fail working with it - such as mkswapfs & mkswapon. For absolute majority of utilities it will work.
-By Alexey Eremenko "Technologov". 21.apr.2006. (updated 12.01.2008)