maximum number of USB hard drive for Raspberry Pi 4

Question

I have a Raspberry Pi 4 (8G) 128 G edition, and try to use a Sabrent 16-port USB hub to connect external hard drive. I installed the latest linux system.

I have successfully mounted 8-9 seagate HDD (each with 10TB-16 TB storage), which works fine. However, while I added more than 9 HDDs, the screen does not show the new added HDD and sometimes all the mounted HDD disappear on the screen, and the computer frozened and then shut down.

My questions are

1. Is any limit of the number and size of external HDD connected?
2. Any advice for the fix?

Thanks.

Answer 1

1. Is any limit of the number and size of external HDD connected?

Most likely. You can only handle so many reading and writing at the same time on one USB port.

2. Any advice for the fix?

Switch out drives you're not using. You don't want to overwhelm the ports and/or the Raspberry Pi. I once had too many things in my Pi 3B and oops... my hub doesn't work anymore.

If you need, Try using lsusb and lsblk to see if the extra ones you are adding. If you don't see them even if they are plugged in, then you may have reached the limit.

Answer 2

The theoretical limit for the standard xhci/USB3 Linux driver is 32 devices. On some systems it's possible to use the old ehci/USB2 driver instead, but not on the Pi. Also, it only makes sense for slow USB devices like serial port adapters, disks benefit a lot from USB3.

The practical limit you have discovered (9 devices?) could be due to several factors:

• electrical power available
• the automount system you are using (udisks2? usbmount?)
• something else

The power issue can be ruled out by measuring the current your hub can deliver, or by connecting 10 thumb drives instead of HDDs and seeing if they get mounted properly. Auto-mount issues can be ruled out by disabling the auto-mount system and mounting the drives manually. AFAIK the Lite version of the OS comes without auto-mount.