Boot with external HD Seagate USB3 only by cold startup

[OSX_rulez]

Hello all, I'm a new MacRumors user
I've read many forums to find an answer to this problem, but even the Apple support couldn't fix it, I hope to find some good suggestions here!

Since High Sierra and now up to Mojave 10.14.5, I can't boot an iMac 5K 27" late-2015 using an external HD Seagate Barracuda 2TB after a restart. I mean that it works perfectly if the iMac is switched off and I do a cold startup but if I need to restart it, eg. after some updates, the HDD is not used as the boot one (so the iMac starts with the internal HD) even if it's correctly set in the Preferences and also it's not listed if I press "option"! I need to switch off and on the iMac or the hard drive enclosure to see it.
This behaviour can cause some troubles especially every time I update macOS and the machine restarts, cause the installation doesn't properly come to an and, and I need to switch off/on to complete the process.
I've tried 2 different USB3 boxes with different cables.
I did a lot of tests with the Apple support, they even sent me a special software called Capture Data to analyse the logs, but at the end they told me "we don't support this kind of issue with external drives"!

HD: Seagate Barracuda ST2000DM006 (it was HFS+ with HighSierra, automatically changed to APFS by the first Mojave update)
Enclosures: StarTech USB3 (SATA III with UASP) S3510BMU33 chipset ASM 1053 (identified by Disk Utility as ASMedia Technology 2105) or Atolla docking station USB3 (SATA III with UASP) CH-326U3S.

Did anyone else see this kind of boot issue?

Thank you!

 

[DeltaMac]

Yes, I have a couple of drives that tend to do that same "some other system, not the one I want" on a restart.
Those drives are both multiple-boot partitions, and a reboot often simply goes to the first bootable partition on the drive. I suspect it has something to do with drive motor speed, as like you have found, it does the same in the other external enclosure that I have.
I just know which drives do that, and I usually will not allow a no-hands restart, and do a restart through the option-bootpicker screen.
Or, I just shut down. I DO see that even that does not always work on one of the drives. I have to cycle the power on that drive. THAT usually takes care of the boot on that drive. And, that also means that I have to shut down, and not try a restart at all.
(The drive that has the most boot problems is a 1TB, new in 2010, so maybe age is related )

 

[OSX_rulez]

Yes, I have a couple of drives that tend to do that same "some other system, not the one I want" on a restart.
Those drives are both multiple-boot partitions, and a reboot often simply goes to the first bootable partition on the drive. I suspect it has something to do with drive motor speed, as like you have found, it does the same in the other external enclosure that I have.
I just know which drives do that, and I usually will not allow a no-hands restart, and do a restart through the option-bootpicker screen.
Or, I just shut down. I DO see that even that does not always work on one of the drives. I have to cycle the power on that drive. THAT usually takes care of the boot on that drive. And, that also means that I have to shut down, and not try a restart at all.
(The drive that has the most boot problems is a 1TB, new in 2010, so maybe age is related )

My external HD has only 1 partition (plus the recovery one) and it was almost new before I connected to this iMac 5K. Before I used it for a few months on a iMac 24" 2008 with El Capitan and it did work, always. It's since High Sierra and now Mojave that I have this odd issue.
Also the 2 USB3 enclosures are brand new.
I don't know what is the difference between a restart and a cold start from the macOS point of view.

 

[DeltaMac]

I've always heard (well, since I began as a service tech nearly thirty years ago) that a power cycle on a computer means that you shut down, so all hardware is powered off. Boot up then fully resets the hardware from a no-power condition, even to the point of unplugging a power cord if one feels that is necessary. A simple restart does not reset completely.
Don't know if the operating system (whatever that might be) cares.
But, from my point of view, a restart is not the same as a full power cycle.

Bottom line here - if your particular hard drive often misses a restart (or maybe not every time), then it only takes one extra step. Shut down, then boot from power off.

Or, if you don't want to trust your external hard drive, replace it. Get a different brand. I prefer W-D over Seagate.

 

[Fishrrman]

Get a small SSD (Samsung t5 or Sandisk Extreme).
256gb could do the job.

Set THAT up to be "the boot drive" (OS, apps, basic accounts but NO large libraries).

Boot and run from the SSD.
The Seagate drive will then be "just another (non-booting) drive" on the desktop.

 

[OSX_rulez]

I've always heard (well, since I began as a service tech nearly thirty years ago) that a power cycle on a computer means that you shut down, so all hardware is powered off. Boot up then fully resets the hardware from a no-power condition, even to the point of unplugging a power cord if one feels that is necessary. A simple restart does not reset completely.
Don't know if the operating system (whatever that might be) cares.
But, from my point of view, a restart is not the same as a full power cycle.

Bottom line here - if your particular hard drive often misses a restart (or maybe not every time), then it only takes one extra step. Shut down, then boot from power off.

Or, if you don't want to trust your external hard drive, replace it. Get a different brand. I prefer W-D over Seagate.

I usually never shut it down, and whenever I need to restart (eg. macOS updates, new drivers) it never shows at boot. And the main contraindication is that sometimes I have to apply the macOS updates twice because when the iMac restarts (with the update still in progress) the process doesn't complete properly. I need to let it boot from the internal HD, shut down, switch on with option key and select the external HD to continue (in this case there is a temporary HD listed called "install" or a similar name).
This is very odd, and what I'm trying to discover is if it's an iMac 5K firmware bug, a macOS bug, or an hard disk related issue. But the same HDD was connected for a few months to another iMac with El Capitan and I didn't have this problem. It began with High Sierra and now Mojave.
[doublepost=1560185256][/doublepost]

Get a small SSD (Samsung t5 or Sandisk Extreme).
256gb could do the job.

Set THAT up to be "the boot drive" (OS, apps, basic accounts but NO large libraries).

Boot and run from the SSD.
The Seagate drive will then be "just another (non-booting) drive" on the desktop.

Hi, this is not convenient, especially if you want a complete Time Machine backup of the whole volume. And I'd rather not have another external disk, there are already 3 of them on my desk

 

[mikehalloran]

Ok. A few things. Apply as appropriate.

If you Option-boot to select the startup disk or partition, you must go to System Preferences/Startup and reselect your startup drive—even if it's showing when you look at it. Don't know why it will show the wrong drive or partition if you don't reselect but it can.

If you have problems with USB drives not always showing on boot up, it can be a dirty connector. Spray a little DeOxIt into the connector, immediately plug it in a couple of times, pull it out, spray again with DeOxIt Gold and wait 2 minutes (these are the official directions). Now reconnect and see if the problem goes away for a few years. I no longer use the spray and find this much easier.
https://www.amazon.com/gp/product/B01BW8VRVY/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1
Believe it or not, this is most likely to affect me with new equipment or peripherals such as the Satech hub I bought a couple of weeks ago—I was ready to return it when I thought, Maybe, couldn't hurt. You'd think it wouldn't...

External drives not formatted APFS cannot see drives that are. The reverse is not true.

Data on externals formatted APFS can be recovered if something screws up if done quickly (as I found out last week). I tried a couple of them—they'll find the files for free then charge you to recover. One took 24 hours and wanted an $89 yearly subscription; this one took 4 hours and was only $55 for a permanent license.
https://www.easeus.com/landing/data-recovery-wizard.htm
As I had backups, I tested only as far as getting the data map.

You cannot boot into the repair partition of an APFS formatted drive if there's another Mac OS installed on any other partition or drive. There are certain utilities such as APFS Snapshots that are only available from the repair partition of an APFS formatted SSD. I have test drives with other OS installed but all are removable—they get disconnected if I have to use Snapshots. This saved me many hours last week after a power outage while I was reformatting a couple of drives.

An HDD is incredibly slow. These should only be used for backup and archival including Time Machine.

 

[Cordorb]

How about when you hold down alt key on a restart ?

 

[OSX_rulez]

How about when you hold down alt key on a restart ?

Hi, as I wrote in my first message "... it's not listed if I press "option"! I need to switch off and on the iMac or the hard drive enclosure to see it".