If you have an M3 Ultra Mac Studio, like me, the answer is NEVER. That’s because macOS 26 Tahoe WILL NOT INSTALL on these machines; you can’t update the existing system nor install it onto an external drive.
I spent HOURS trying to figure out what I might’ve been doing wrong, and I just found this page:
https://discussions.apple.com/thread/256136192?sortBy=rank
Everyone there has an M3 Ultra Mac Studio, and like me, they are all experiencing the same problem: whether installing onto another drive or trying to update the internal drive, the process completes and then restarts back into 15.7 again. Logs show an Apple Neural Engine “hardware failure,” where Tahoe is trying to call a driver that isn’t there.
BTW, I used the same installer and the identical drives on my M4 MacBook Pro, and everything went without a hitch.
I called Apple about this, but the guy I spoke with didn’t know anything. I also filed a report, but I’m posting here in hopes of getting the word out on this and putting some pressure on Apple to fix it!