iMac external boot issues

[Ndaa75]

Hi all,

Experiencing some rather odd behaviour from my mac this week.
ive been booting my system using an external SSD drive without issue for over 2 years. During this time ive also had connected, via USB, a 4TB drive for time machine back ups & a dual SSD caddy. Never had an issue & have been really pleased with the performance. Then all of a sudden the Mac wont boot if all drives or 2 drives are connected at the same time & i end up with the '?' screen. Disconnecting the back up drive & the caddy will allow the mac to boot as normal & i can plug them back in once the OS has loaded & all is fine. I've checked cables, run diagnostics, reloaded the OS, changed disks, performed the SMC and NVRAM reset procedures & checked the ram modules all to no avail. I'm wondering whether theres an issue with the USB or some other hardware problem. Just seems odd it was working fine and all of a sudden this problem. Havent changed anything at all on the system & am completley baffled. Is the Mac on its way out? Anything else to try?
Mac is a late 27 inch 2013 model running Catalina.

 

[Fishrrman]

Just some thoughts...

I'd check the startup disk preference pane and re-designate the external SSD to be the boot drive.

If that doesn't change anything...
... what happens when you boot while holding down the "option" key to invoke the startup manager?

You should see the external SSD as a boot choice.

If nothing above is helping, you might try disconnecting the dual caddy (dock).
I've heard that these can sometimes cause problems with Macs (something to do with the USB control chip?)

 

[4sallypat]

I have been using a Thunderbolt SSD external LaCie and Transcend as boot drives for a year on my 2013 iMac without a problem.

I prefer using TB port as it is faster than USB....

 

[Ndaa75]

Yep, tried all of that including checking start up preference & option on start up.
The Mac will only boot from the external drive if no other drives including non powered disks are not attached via USB. Just seems very weird. Wondering if there's somthing wrong with hardware when it boots.
Noticed yesterday too that if I try to boot with say 3 drives attached, it wont load but if I turn the computer off & on really quickly its finds it an boots normally.
Not sure if there are any logs that I can check or terminal commands to reset the USB bus perhaps.

 

[Ndaa75]

I have been using a Thunderbolt SSD external LaCie and Transcend as boot drives for a year on my 2013 iMac without a problem.

I prefer using TB port as it is faster than USB....

Never had an issue until this week. The cost of Thunderbolt put me off.

 

[Nerdy Norm]

Did you find a solution to this? I've got exactly the same issue. The iMac boots perfectly with the external USB SSD as long as no other drives are connected. With another USB drive connected, it always boots to the original (very slow) internal drive. To stop this happening, I wiped the internal drive thinking that this would force the Mac to use the next available boot disk. However, all that happened is that now it doesn't find a boot disk at all, just giving the flashing question mark. If I unplug the other USB drive, it boots fine again.

 

[Fishrrman]

Norm wrote:
"To stop this happening, I wiped the internal drive thinking that this would force the Mac to use the next available boot disk. However, all that happened is that now it doesn't find a boot disk at all, just giving the flashing question mark. If I unplug the other USB drive, it boots fine again."

What happens when:
- you power off
- connect the other drive
- power on and IMMEDIATELY hold down the option key CONTINUOUSLY until the startup manager appears?
- can you see the external boot drive?
- if so, can you click on it, hit return, and boot from it?

Could it possibly be a conflict between USB drivers or controllers in the other drive which conflict with the external boot drive...?

 

[Nerdy Norm]

Thanks for the input. I have (temporarily I hope) attached the USB drive to a Synology NAS and it's about 25% through its first TM backup. I don't want to interrupt it until it's complete, which seems like it'll be a couple of days. Once that's done, I'll try out what you suggest and report back.

 

[Fishrrman]

Norm wrote:
"I have (temporarily I hope) attached the USB drive to a Synology NAS and it's about 25% through its first TM backup. I don't want to interrupt it until it's complete, which seems like it'll be a couple of days."

Egads, man, what is the size of that drive?
It shouldn't take "days" to backup.

A better way:
DON'T USE "NAS".
Use another USB3 drive.
Use CarbonCopyCloner or SuperDuper to "clone" one drive to the other.
Should go much faster.

Both CCC and SD are FREE to download and try for 30 days.

 

[Nerdy Norm]

The iMac's SSD has around 1TB on it and I guess the wifi network is slow! I want to use TM as it's my wife's computer and I need something that just runs in the background without needing attention. Hopefully I'll be able to connect the USB3 drive directly to the iMac instead of the NAS once I find a solution.

 

[Ndaa75]

Did you find a solution to this? I've got exactly the same issue. The iMac boots perfectly with the external USB SSD as long as no other drives are connected. With another USB drive connected, it always boots to the original (very slow) internal drive. To stop this happening, I wiped the internal drive thinking that this would force the Mac to use the next available boot disk. However, all that happened is that now it doesn't find a boot disk at all, just giving the flashing question mark. If I unplug the other USB drive, it boots fine again.

Yes, I did.
Backed up everything, reformatted everything & reinstalled Catalina. Works fine now.
Obs a conflict somewhere but OK now.

 

[Nerdy Norm]

Yes, I did.
Backed up everything, reformatted everything & reinstalled Catalina. Works fine now.
Obs a conflict somewhere but OK now.

Thanks - not sure I can face going through that again but glad it worked out for you!