Samsung T5 External SSD Startup Disk Periodic Error

I have two T5s running Mojave and I have never seen this message. I use APFS with GUID with no problem.

Use safe mode to isolate issues with your Mac - Apple Support

You might also try Etrecheck to see if it finds anything.

https://etrecheck.com

Does resetting also SMC help?

https://support.apple.com/en-us/HT201295

What about inspecting and resetting EFI partition? The easiest way seems to be reinstalling the system. Do make good backups before any of this.

http://osxdaily.com/2016/07/28/fix-broken-efi-partition-mac/

Or potentially dangerous Terminal commands:

https://www.macobserver.com/tips/quick-tip/macos-removing-windows-efi-boot-entry/

https://apple.stackexchange.com/questions/57597/how-to-fix-broken-efi-partition/58892

Check my post at (link does not work well, make sure to page down to my actual post (ianieto))

Sep 1, 2019 10:47 PM in response to dwb

In response to dwb

External SSDs might not work with the Mac mini 2014 after an update to Mojave.

If you had a Windows BootCamp partition on your internal HD, then you may have issues when you clone the drive. It is very difficult to clone a Windows BootCamp partition and I don't recommend it unless you know exactly what you are doing (even then you may still have issues). You are better off erasing the external drive and cloning your macOS volume including the hidden recovery partition to the external drive. If you still want Windows BootCamp, then use BootCamp Assistant to resize the drive and reinstall Windows.

If cloning just the macOS portion still causes issues, then erase the external drive and reinstall macOS cleanly then migrate you internal drive's macOS user accounts, settings and apps to the external drive.

Dual booting is always difficult and both Apple and Microsoft never make it easy. You are much better off running another OS in a Virtual Machine if possible. Some virtual machines are free.

I didn't bother responding to your post there but I guess I'd better here. Mojave isn't causing wide spread problems with external SSDs. The head of the tech department at the university thought he'd do me a favor and get bigger drives instead of the 256GB upgrade I requested for my lab and office so I got stuck with 3 rotational drive computers that cannot be easily upgraded internally. So I've been running these computers for several years now through several different OS versions without any speed hits due to Mojave or any other OS upgrade. There are however known issues some bus powered enclosures and Samsung's T5 external drive is one. There is a slight delay at powerup that can lead to the drive failing to boot the computer.

I am thinking the problem I saw on my Mac was only related to my 2014 Mac mini because during my debug with Apple support, I did try my Samsung T3 500GB drive on a 2015 and 2017 MacBook Pro computers and it worked fine. Please note that this is an issue only with booting to the external SSD AND when at least one or more other drives were connected during boot. Really a strange and specific problem. The SSD drive was not even detected, with me trying the Option boot idea too. Since it was likely not very common problem and just for an older machine, Apple probably did not want to bother fixing the boot firmware, of which I was very disappointed in them not caring.

I had this problem on my 2013 Mac Pro which only had Mojave installed. I did have a bootcamp partition a few years ago. I had completely wiped my drive since running Windows. On booting it would show the Windows error screen with my T5 directly plugged in until I selected my Mac startup disk in the control panel. Only the Mojave startup partition was visible. My T5 runs perfectly now.

Try booting directly attached and without a USB hub.

The Samsung T5 is a great data drive but there are known issues with using it as a startup drive. It is believed that the problem is due to its being slow to power on. See this article.

Make sure you select the Mac startup volume in the Startup Disk control panel. It will then load into Mac correctly. I had the same problem and this fixed it.

I like using Carbon Copy Cloner to clone bootable macOS drives as it is simple to use and just works. It will also recreate the hidden recovery partition.