2025 Macbook Pro 16" M4... having a hard time setting up external drive to boot from.

[Turnpike]

I've always used an external drive instead of the factory internal drive as my main drive on all my Macs over the years... Yesterday I got a new M4 MBPro and can't get it to boot from that drive. Every time I try, it says "the OS on the external drive needs to be reinstalled" or something like that, which I have done 4 times, starting from scratch, from total reformatting and reinstalling the OS.

I've looked online and have only found others asking the same question, is there a solution to this? Since I'd be putting so much stuff on that drive, I'd like to make sure it's done right and that I understand what's happening, is there some extra step for the Mac M series Macs that I am not realizing?

Thanks in advance.

 

[Fishrrman]

"I've always used an external drive instead of the factory internal drive as my main drive on all my Macs over the years..."

The time has come to change your way of thinking.

The internal SSDs on the new MacBook Pros are VERY fast -- probably considerably faster than most (nearly all?) external SSDs that you might use.
What is the speed of your internal SSD?
What is the speed of your external SSD(s)?

Another consideration is that with the m-series Macs, Apple has made it considerably more difficult to create, boot from, and maintain external boot drives. It can still be done, but it's more work.

Personal experience:
I just created an external boot drive with the Tahoe public beta a few days ago, using my m1pro MacBook Pro. It works, but it's really only "for experimentation" -- not intended to be my main, working drive.

I'm thinking (guessing, really) that in the next few years, Apple may completely disable external booting for the OS (for whatever reasons they may have).

You also wrote:
"Since I'd be putting so much stuff on that drive..."

If you have that much stuff -- to the point where it just won't fit on the internal -- the solution is to keep "large libraries" and other stuff that is seldom-accessed (but still eats up drive space) on an external drive, but STILL BOOT FROM the internal.

The internal should have:
OS
applications
accounts (but leave large libraries elsewhere)
frequently-accessed data

Years ago in this forum, back in "the HDD days", I used to advocate booting from an external SSD instead of the internal drive. I ran my 2012 Mini that way from the day I took it out of the box until I retired it, quite a few years later.

But... those times... are over.

Again, it makes more sense now to boot and run from the internal SSD.
And, I reckon that someday before too long it may become the ONLY way that the Mac OS will run...

 

[Turnpike]

"Those times... are over." Wow. I had no idea. I just bought a brand new MBPro 16" and got the 48GB RAM instead of more storage, I didn't realize this. I should have at least got the 1TB, but Apple makes it an almost criminal price increase for RAM and storage, I figured I'd save on the one, and invest in the other. Well, then it's time I learn a new way, thanks for the info, getting this reply from someone who knows helps a lot in my relearning how I use my Mac and store my files. I appreciate you typing out that full reply. 👍

 

[Fishrrman]

I'd suggest at least 1tb of internal SSD.
How much did you get?
Is it still within the return period?

 

[Bigwaff]

Yesterday I got a new M4 MBPro and can't get it to boot from that drive. Every time I try, it says "the OS on the external drive needs to be reinstalled" or something like that, which I have done 4 times, starting from scratch, from total reformatting and reinstalling the OS.

Post the brand and specs of your external drive. Thunderbolt? USB4? USB3?

 

[ISKOTB]

"Those times... are over." Wow. I had no idea. I just bought a brand new MBPro 16" and got the 48GB RAM instead of more storage, I didn't realize this. I should have at least got the 1TB, but Apple makes it an almost criminal price increase for RAM and storage, I figured I'd save on the one, and invest in the other. Well, then it's time I learn a new way, thanks for the info, getting this reply from someone who knows helps a lot in my relearning how I use my Mac and store my files. I appreciate you typing out that full reply. 👍

I agree with Fishrrman return it if you still can and get at least 1TB of internal storage.

To boot from an external HDD or the SD card reader check https://apple.stackexchange.com/que...cos-from-an-external-device-on-macbook-pro-m3