Pi4: Can you use two USB3 add-ons at once?

[rossi46]

I ask because, my Pi4 doesnt like it when I plug, say, a 32gb flash drive in while my 250gb ssd is already in the other USB3 port. The ssd unmounts and remounts itself constantly when I do this. If I plug the flashdrive into the usb2 port, it's fine and I can slowly transfer files to my heart's content.

I have the 4gb Pi4 with pi-coupe case, fan shim, official psu, official keyboard and mouse and ssd running beautifully.

All is fine until I insert something into the spare usb3 port. Is this normal?

[rpdom]

The power supply to the Pi's USB ports isn't quite the same as a PC with a large power supply behind it.

Plugging anything into a USB port while the Pi is running can (not will)cause the supply voltage to drop (even briefly) below the level where the Pi is stable.

It is best to plug any devices into the Pi's USB ports before you plug the power in.

[FTrevorGowen]

The power supply to the Pi's USB ports isn't quite the same as a PC with a large power supply behind it.

Plugging anything into a USB port while the Pi is running can (not will)cause the supply voltage to drop (even briefly) below the level where the Pi is stable.

It is best to plug any devices into the Pi's USB ports before you plug the power in.

+1. W.r.t. the OP's main question, quick answer, Yes, but it probably depends on what the USB3 "add-ons" are, and their "current needs" ("MxPwr" reported by usb-devices). For example I have used a USB3 flash drive** and USB3 uSDHC card reader/writer** successfully (with keyboard & mouse connected via a USB2 switching hub into a USB2 port):
http://www.cpmspectrepi.uk/raspberry_pi ... uSDHC_card
Trev.
** More info. here: http://www.cpmspectrepi.uk/raspberry_pi ... lash_Drive