free space on ubuntu mount drive disappeared

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http://askubuntu.com – I came back from vacation, inserted my ubuntu flash drive into server and fired it up, only to find out that df -h reports 0 free space. 100% full 3.8GB drive. I installed ubuntu server on flash usb key, and I'm pretty sure it was half empty before leaving. Plus I had it unpluged during my stay out so no hacking could take place. Now I wonder what happened with free space? corrupted? damaged? temp files? what is it?? I also found that history wouldn't show me any history... on that box. weird. (HowTos)