Mono Search and Rescue got called a few days in the early afternoon and we finished up around 0500 the next day: roughly 14 hours of humping and pumping through ice and snow, waiting around, treating patients, and rigging rescue lines/anchors/stuff. After some sleep I checked the back of my truck:

SAR packing seems to be based around three key concepts: mandatory, optional, and spare capacity.
Mandatory Gear
Bivy sack, head lamp, dry socks, and things of that nature: stuff that you’re going to carry no matter what. Most of those items are specified by my team but I toss a couple of my own in there too. I’m sort of a dork so I wear my helmet to almost anything except blazing sun mid-day and my own medical go-bag.
All of this stuff is in my pack, ready to go.
Optional Gear
Next to my pack is a milk crate full of optional items. My tent (as opposed to my tiny bivy), my backpacking cooking gear, crampons, snow shovel, helicopter goggles, etc.
The reality is that I can’t bring everything you’d ever want so on a operation-by-operation basis an educated guess is made. Headed to the desert? Extra water. Headed up a snowy mountain? Snow anchors.
Spare Capacity
Another reason you can’t load up on everything you want from your own gear is that team gear has to be lugged out as well. The litter, oxygen tanks, vacuum splints, long ropes, and all nature of climbing and medical equipment beyond your personal gear needs to head out as well.
And remember that you need to balance all of that with the fact that the more you’re lugging the slower you go and the less nimble you are. With more gear you get to your patient(s) slower and your agility at transporting them out is reduced.

I’m still relatively new in my SAR career so I imagine I’ll be learning and refining as I go. And then there’s winter, which in a place like Mammoth really goes haywire. My own personal gear is much bigger, overnight gear is bigger, and between skis/boots/crampons/splitboard/snowshoes it’s tricky as hell to walk around.