Default [Q] After flashing SlimKat space still occupied...

After I flashed SlimKat ROM onto my Galaxy Tab P6200(16gb) the space available is only 4 gb and other 8gb is used by something.Before flashing I messed around with Titanium Backup and I backed up all user apps+data but when I did that I only had like 1 gb left,the backup terminated saying I had to free up space well I didn't try to back up again and my space was full.After that I tried to flash and yeah 8gb is being used by something I don't know,it's hidden somewhere I guess.
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