OptiPlex 390 won't boot with USB HD attached - 2 3 indicator LED

There is no legacy mode because the 390 is class 1 legacy bios. Its legacy all the time. Its not UEFI.  USB3 devices try to draw too much power from the bus.

2 3  means hard drive failure.

https://www.dell.com/support/article/en-us/sln284978/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators?lang=en#2009_to_2012

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@GaGirrl63 wrote:

I have a Win10 OptiPlex 390 with a Seagate Backup BUP 1TB drive attached. Until recently, it booted fine with the drive attached, but recently, it won't unless I unplug it until after POST.

I've updated the BIOS, I've disabled USB boot devices, I've moved the drive to another USB port -nada.  The active flag is not set on the USB drive partition so it's not bootable.

The only other suggestion I've seen is to disable USB legacy support; however, if I have to go into safe mode, I presume USB peripherals devices don't work?

Any other suggestions? Lesser of two evils? Unplugging the USB drive on boot or disabling USB legacy support ?

Or is there a third option?

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Seagate's only other suggestion that you did not mention is that the drive is formatted NTFS.

https://www.seagate.com/support/kb/computer-wont-boot-with-usb-drive-connected/

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The drive inside needs 1full amp to start up the motor inside

The fails on vast PCs, (or is marginal on others (yours sure)

The USB2 PC you have , has vary limited current . even 500 or 900mA limited, the latter is super marginal.

solutions. are many I will list them all.

1. Buy and use only external SSD drives. USB.
2. Buy only HDD drives with external power pack, (matched by maker to ITS need)
3. Buy a full powered and buffered USB hub , (with power pack matched, by maker)
4. Buy  PCI-express card called USB  3.1 or USB-c rated over 1amp on the ports.  Newegg.com has em'

if you learn to put ear to the casing of HDD , you not hearing the motor start up tells you (POWER IS BAD)

or worse, like mad loud clicking of HDD, inside. (power is weak, drive boot up, gone mad)

others posting read  the OP stated, the PC will not boot with USB STHH1000400  , hdd plugged in.

this  is power issue for sure if hdd does not spin up . (heard)

the secret this toy drives is seen at youtube, (nice and rare dismantle)

if you watch the video you see what is inside, and it is a real HDD, and that drive and others all need a full

1 amp motor startup or the drive is born dead, (on your PC dead or marginal)

learn to listen to your drive, ear to case, hear it fails to start the motor and keep it running is POWER FAILURE

simple and true. 

NO PC really told is this MT, DT, SFF? ? 390MT/  (case form factor)

all HDD made PARK THE HEADS, so that it does not crash the heads at power off and on.

when turned on the motor starts up  (if the 5vdc pin is not too low , that is) 5vdc VBUS USB in this case.

then when and if the motor reaches, full spindle RPM the brain inside the HDD sees that and unlocks the heads.

The heads are now flying,  (Bernoulli's principle)

The head servo then responds to the BIOS, identification (pnp) command and I stop here. (lazy me)

if the hdd never spun up then power is LACKING. (use your ear to the case here the motor spin up or not)

@nyc10036  the PC works fine without the external USB drive attached and it is NTFS formatted.

I've had it for a while so returning it isn't an option - I also use it to sync between my desktop and laptop (which I use on the road).

@savvy2  , I do like the last few options of getting a case and new SSD drive or a better SSD external drive, now that I understand that my current system is under-powered.

Thanks all.

@GaGirrl63 

Getting an external SSD drive is probably best going forward, but they will be pricier than the traditional external hard drives

Samsung 1TB is $179 https://www.newegg.com/samsung-t5-1tb/p/N82E16820147641

@nyc10036 

It can't be faster than savvy's seagate. Say it ain't so!