Preview or photos? These are different things:
The previews - normal and smart - are independent from the original photos. Deleting any of the previews will never harm an original photo and previews can always be recreated by Lightroom.
Deleting a photo gives you the option to only remove from the catalog and leave the original photo alone, or remove from disc also.
Thanks F.McLion. I understand that photos and previews are different things. When I make an edit to a photo using the smart preview the original gets update when I reconnect my hard drive (I know that these are non-destructive edits) I guess another way of posing the question is that if I flag a smart preview as 'rejected' when I connect my hard drive does that flag sync with the original photo?
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Yes, the flag and any other metadata will apply to the original.
Thanks, John, so flag but don't delete until hard drive reconnected.