I actually really like Transmission, except for that one (IMO) major problem.

To be clear, here is the issue, spelled out:

If you have a Torrent that contains multiple files, you can tell it to only download a particular subset of them. Chances are, however, that a torrent block at the beginning and one at the end of the file you're interested in cross over into the previous and next files, respectively.

The correct thing to do here, IMO, is to either store that extraneous data in a cache somewhere or just throw it away (which means that you would no longer be able to provide those blocks to other peers).

What Transmission does, though, is write it to the deselected files anyway. First off, that's just irritating. I don't want partially-written files lying around my filesystem pretending to be valid.

But, more importantly, the MacOS filesystem, HFS+ will allocate space for a file up to the last byte written, even if there is no data written prior to it. So if the file in the torrent before the one you're interested in is 500MB, even if Transmission only writes a few kilobytes at the end of the file, all 500MB get used up.

Even better, once you go and delete those files out from underneath Transmission (and it's not exactly trivial to find those partial files), it notices that those blocks you partially deleted are now corrupt, so it downloads them again.

I've submitted bug reports to them about this, but they seem mostly uninterested in resolving the problem.