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macrumors newbie
Original poster
Apr 27, 2010
I'd like to run something past you guys. I have a MBP (Retina, 15in, Mid 2015). It's internal drive is 10.14, and I have a external USB drive partitioned, one for 10.15, the other for Bug Sur. I usually run 10.14 but I need to boot into the others for small reasons.

Recently I haven't been able to boot into Big Sur, the Mac hard crashes and reboots. (Tried most everything, usual suspects, doesn't work.) With some experimentation, I found that I can SAFE BOOT and that works, but of course the Mac runs very slow.

Another thing I found was my MBP's battery is pretty much done, which I rarely notice because I always have it plugged in.

I just got a new external USB drive, and I partitioned it the same way and installed 10.15, which was fine, then installed Big Sur on the other partition, and it did the exact same thing - I can SAFE BOOT but not real boot.

So, sure, I'm going to be my MBP serviced and have a new battery put in... but my theory is...

Big Sur has some dependency on the battery, even if the MBP is plugged in, with (at least) external USB system drives to real boot, and apparently the safe boot avoids this dependency. Sound reasonable?
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