I frequently use multiple USB 3.0 devices on my Pi 4B2 and 4B4 and have not had any trouble with wired devices.
USB 3.0 is known to cause interference with 2.4GHz wireless devices, which is a common frequency for wireless keyboard and mouse (not to mention, Bluetooth and WiFi). So if you are having issues with a 2.4GHz wireless keyboard/mouse dongle, try putting it on the end of a short extension cable and moving it away from the Pi4 (or you could try moving the USB 3.0 devices away).
As far as storage devices go, I frequently connect a USB 3.0 card reader, or even another SSD to the other 3.0 port for making backups of my OS SSD with the SD Card Copier utility. I've done that on the older setup with /boot on SD card, and with the new beta USB bootloader, and it works fine. I even ran a 4B2 from dual SSD for awhile as an experiment (/boot on SD card, / rootfs on one SSD and /home on the other).
Some of what I'm reading sounds like power issues. The Pi4B's USB ports are limited to 1.2A total for all 4 ports, like previous models, which is far below the 900mA per port of the USB 3.0 power spec. However, there are no per-port limits, so all 1.2A can come out of 1 port when it's the only port in use. Connecting HDD and SSD could easily surpass that limit in some cases, which is why the poster above could not boot with both connected (hot plugging it after booting reduces the load, since the SoC and boot drive are mostly idle at that point).
I recall once getting dual HDD mounted on a Pi3, but it wasn't a reliable or usable system. Any action that used both would result in disconnects and failure (like trying to copy files from one HDD to the other HDD, although either could be used individually). Dual SSD, sure, but not HDD.
If you want to use dual HDD reliably, you need to power them externally (even SSD + HDD is iffy, it might work with some drive combinations, but not all). Dual SSD should work with modern, power efficient SSD models (some SSD models have shockingly high power requirements).
LTolledo wrote: ↑
Fri May 29, 2020 10:08 pm
One thing I learned from this experiment is NEVER PAIR
any Seagate HDD with a bus-powered dual HDD setup on the RPi4B connected at bootup.
LOL, yea, Seagates are hungry little buggers. The WD Blue hard drives are pretty frugal (I have several, and they're all rated at 550mA).
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?