The Pi3B+ can supply up to 1.2A total across the 4 USB ports. See Raspberry Pi Power Limitations.
This assumes the Pi power supply is adequate.
Whether this would be adequate for 2 SSD drives, depends on the drives. My experience is that the voltage from the USB ports often falls below that required for power hungry external drives, so a powered hub is desirable.
For connecting two USB HDDs, does an Rpi USB Hub need any external power?
Well, it also depends on your HDD’s power requirements. For reference, let me describe how I am using one 2.5″, 5,400 rpm, 2TB SeaGate Backup Plus Model SRD00F1 HDD, as shown in the picture below.
The popular Seagate Backup Plus HDD series has 1/2/4/5TB versions, and power requirements is within 2W. In other words, the maximum current is around 500mA.
I first tested the HDD using an USB hub without external power, on both Win10 PC and Rpi3B+. I found the following current measurements.
- Idle current ~= 170mA
- Startup and working current ~= 200mA ~ 380mA
I mainly use the HDD for backing up my Rpi Micky Mouse project’ python programs, with a total size of much less than 1MB a week. In other words, I only use the backup HDD for perhaps 10 minutes each couple of days. I mostly use an 16GB USB stick for frequent everyday backup, perhaps 10 times an evening.
In order not to draw unnecessary power from the Rpi USB power source, heating up the Rpi, I am doing the following.
- I am using a 4 channel USB hub with individual on/off switches, and I switch on power for the USB HDD only when I am going to do backing up, therefore saving 170mA idle current.
- I also don’t automatically mount the HDD during boot time. I only mount the HDD before backup up, and unmount immediately after backing up. This way avoids carelessly unplugging or freezing Rpi, corrupting the HDD.
I found using one Segate 2TB HDD using USB Hub without external power is good enough for my hobbyist projects.
Of course if I am doing more serious work, using more than two 5TB HDDs say, I will definitely use an externally powered hub.
/ to continue, …
Appendix A – Newbie friendly Rpi HDD config notes
Part A – Partition and Format USB Hard Disk
Chinese Windows 10 > Control Panel > System and Security > Partition and Format Hard Disk
Part B – Rpi External Hard Disk Installation
# List partitions on hard disk $ sudo lsblk -o UUID,NAME,FSTYPE,SIZE,MOUNTPOINT,LABEL,MODEL # To find locaction hard disk partition (would find /dev/dsa1) $ sudo blkid # Crete mount point name "mydisk" $ sudo mkdir /mnt/mydisk
Part C -To set hard disk permissions
$ sudo chown pi:pi /mnt/mydisk $ sudo chmod 777 /mnt/mydisk
Part D – To mount and umount hard disk
$ sudo mount /dev/sda1 /mnt/mudisk $ sudo umount /mnt/mydisk
Part F – Install exfat and ntf drivers
$ sudo apt-get update $ sudo apt-get install exfat-fuse $ sudo apt-get update $ sudo apt-get install ntfs-3g
/ to continue, …