I sometimes update the underlying file outside Photoshop:
Existing guides in Photoshop are then lost by default. How can I keep the existing guides when the underlying file is updated in Photoshop?
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This is how I would do it using a Smart Object
Do File > Open as Smart Object, navigate to the jpeg image you want to use, select and open it.
Add your guides.
Save as PSD.
Now if you update the jpeg outside of Photoshop, the guides will still remain in the PSD. You will have to open the PSD to see them.
Note: If the smart object doesn't update automatically after you made a change outside Photoshop, then right click the Smart Object in the layers panel and choose "Update Modified Content".