PS is writting on disk C during Stack Mode process instead of on scratch disk

There are some functions in Photoshop that use the operating system pagefile for temp storage, not the scratch disk. I didn't know about this particular one, but smart objects, some of the new AI-based functions, and anything processed in the GPU will do this. The scratch disk mainly stores "finished" history steps.

The scratch disk is purged when Photoshop closes. I'm not sure how the pagefile works in this regard, but considering that it's temp storage, I assume it's cleared when no longer needed. It will in any case be deleted on reboot, so there's no need to delete manually. The basic pagefile is 2x installed RAM (IIRC), but dynamically allocated and will expand as needed.

All that said, I would take that warning seriously. It means you really are short on disk space. Raster image editing moves huge amounts of data around, orders of magnitude more than any RAM you may have installed. It all has to go somewhere, and you need to make space for it. People usually blame Photoshop for this, but it's really just a lot of data that needs to be handled one way or another.

Cleaning out your system drive can often free up hundreds of GB. There's free software that will show you exactly what fills up your drive and where it is. I'm using WinDirStat out of habit, but it's slow and I understand there are som faster alternatives:

in the future, to find the best place to post your message, use the list here, https://community.adobe.com/

p.s. i don't think the adobe website, and forums in particular, are easy to navigate, so don't spend a lot of time searching that forum list. do your best and we'll move the post (like this one has already been moved) if it helps you get responses.

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