A photographer's Mac fills predictably, and it is fixable by workflow

Photography fills a Mac in a predictable order: originals, then previews, then exports, then the copies that exist because two applications each wanted their own. Only the first of those is irreplaceable, which is what makes this fixable.

Where it goes

The setup that works

1. Originals on an external drive, organised by date, with a second copy somewhere else.
2. Catalog on the internal drive, because it is small and constantly written to.
3. Previews sized to your screen rather than 1:1 for everything, and set to discard after 30 days, per Lightroom previews.
4. Exports treated as disposable, deleted after delivery.
5. One library, not two, because a Photos library and a Lightroom catalog holding the same images is two copies.

The Photos and Lightroom overlap

Many photographers end up with images in both, because a phone syncs into Photos and a camera imports into Lightroom. That is two complete copies of anything shot on both. Deciding which one owns which images, once, is worth more than any cleanup, and what the Photos library holds covers its side.

Culling is the only real answer to originals

Previews rebuild and exports re-export, so the only category that grows without limit is originals. A culling pass at import, deleting the obvious failures before they are ever backed up, compounds over years in a way no cleanup does.

What to check when the disk fills anyway

du -sh ~/Pictures/* 2>/dev/null | sort -h | tail
find ~/Pictures -name '*Previews.lrdata' -exec du -sh {} + 2>/dev/null
du -sh ~/Pictures/*.photoslibrary 2>/dev/null

Nine times out of ten the answer is previews or a second library, both of which are dealt with in an afternoon. If the answer is genuinely originals, what moves safely to an external drive is the next step.