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I switched from uTorrent to qbittorrent some weeks ago and I'm somewhat concerned about what I just found and cannot explain.
qbittorrent seems to be filling my HDD for no apparent reason and I cannot get my HDD space back.

I have one active torrent, all other are paused. This torrent is ~22GB in size and qbt took this space like it should - Windows Explorer shows this amount of size for all the files as intended.
The problem is, that qbt continues to allocate disk space. The interesting thing though is, that when using "Process Monitor" to see what files qbt writes I see absolute nothing special - no temporary files or anything just writes to the torrent dl files as it should be.
Yet the used disk space goes up by a lot more than the size of the torrent files (which in all should not change anyway, but they do).

For example see the screenshots below, it shows the size of the files at the bottom and the Windows disk manager free HDD space at the top (I cut out not relevant data):

https://imgur.com/a/h40ay

File size grows by 4.194.304 bytes
HDD used grows by 865.779.712 bytes
This is 206x the size! And the second screenshot was after closing qbt. And no there is nothing else going on, the used space stayed like this after closing qbt for quite some time.

How? And where is my free disk space gone? And more importantly, how do I get it back?

Are you using sparse files or preallocate files in qBitTorrent?

I used sparse files but now switched to preallocation.

I found the problem. The Windows partition info "used space" shows the actually used space on disk - but the Explorer (and the program I use as the Explorer replacement) show the "file size" but not the "size on disk". So the value in the Explorer stays the same all the time, but the space was not allocated at all and gets allocated when downloaded. See example: https://imgur.com/a/9dC7L

I hope this explains is (because I found some other reports here on the board where people downloaded like a 50GB file on a 60GB disk but qbt showed as not enough disk space, so I imagined there might be some bug, hence my post).

qBitToerrent may warn of being out of disk space and even stop downloading before disk space is 0, to avoid even worse problems.
It may need a tweak on how that works...