It's an important matter
Photos I've edited, delivered, and have no further plans for, I automatically archive. So imagine my alarm when I discovered that the external drive didn't have enough free space! To be precise, there was plenty of space — just on the wrong partition. Which meant moving data around so I could expand the partition reserved for photos and give it enough room. I created twice the space, which should hopefully last a few more months. I also realised I don't need to archive everything — edited photos intended for the web, or archives meant for web downloads. That'll save some space. What do I actually archive? NEF + the corresponding XML file describing the edits for exporting from that format, and PSD with all layers and adjustments at the highest possible resolution.