As the output of df -h shows here, something is eating up 5GB of free space. So, it's not available to use. I'm also noticing sometimes that the hard disk gets filled up to 100% sometimes. So, I had to restart the machine or delete some unncessary files. Only noticed these in /home partition. Don't know whether these two are related, but appreciate if anyone can put some insight into this.
$ df -h Filesystem Size Used Avail Use% Mounted on /dev/sda8 100G 92G 2.7G 98% /home