Question My External drive just filled by himself

Feb 24, 2019
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Hello people,
Yesterday I cleared about 70 GB of old shows that I already backed up from my external hard drive. Now this morning I look and it shows only 15,5 GB available.
I didn't do anything myself in the mean time, does anyone know what this could be?
Or how to free up more space? I see some defragmentation things on google but I'm not sure if that's the answer since this space was filled overnight
also, the totall space is 4 TB (shown as 3,63 TB but i guess that's standard)

What do you recommend?
The data currently on it is not otherwise backed up
 
I got SpaceSniffer from a friend, looks just the same, does the same thing as WinDirStat
Though it does help to see what uses what space, It doesn't show me what happened over night, wich was last modified and such