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If you have the file still open, you should be able to overwrite it's contents. Of course, you do have to have opened it for writing. If you're trying to simultaneously open the file a second time for writing and change its contents.. well.. that way lies bugs, which is why you're not allowed to do such dumb things.
PhotoShop refuses to write to my NAS. I have to store locally, then move the file.
If your operating system has no problem writing to the NAS then but PhotoShop cannot... then I would suspect that PhotoShop is using its own SMB implementation. It may be attempting a SMBv1 connection[^] to your network storage device.
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.. I would suspect that PhotoShop is using its own SMB implementation
Photoshop doesn't have an SMB implementation built in, that'd be silly. More likely, is that the NAS is using ext3 and it's SMB implementation isn't mapping all the SMB features to the foreign filesystem (or the foreign filesystem can't support them). If Photoshop is using one of those SMB features, then it might indeed faila write, not because Photoshop is deficient, but rather because the NAS is deficient. I've had to debug that exact issue before, twice, albeit not with Photoshop.
No save as options? No way of copying it to the clipboard, and save it using MSPaint?
It's a bug btw, due to lack of understanding; if it had been tested to see if basic functionality is at least available, it would have been caught. What buggy, untested software are you using if "Save As" and copy to clipboard aren't even available?
You do know that there is a free, open OS that has some decent image-editors, also for free?
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