RPi Adding USB Drives

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Revision as of 20:30, 13 July 2012 by Prpplague (Talk | contribs) (moved Adding USB Drives to a Raspberry Pi to RPi Adding USB Drives: change title to fit with naming scheme)

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USB Flash and Hard Drives

Sooner or later, you're going to run out of room on the SD card used to boot and run your Pi. One solution is to move to a larger SD card, but, the largest low-cost high-speed cards are 32 GBs for about US$20 ~ $45 (May 2012). However, there may be compatibility issues between these higher-capacity SD cards and the Pi, at least, temporarily. USB flash drives (aka "thumb" drives) can provide additional storage for about the same, or less, cost as an SD card of the same capacity. USB flash drives generally also have faster data transfer speeds than SD cards. Another option is to connect a USB hard disk drive, which can provide up to over a terabyte (1 TB) capacity and data transfer speeds well beyond 40 MBs/second, much larger and faster than SD cards or USB flash drives.

Powered USB Hubs

In any case, in order to expand the storage capacity, you will need to connect a powered USB hub to one of your Pi USB jacks, and then connect the USB drive to the hub. A USB hard drive can either have its own AC/mains power supply or it can have two USB plugs, and get a USB 2.0 hub with at least four expansion ports - more is better, within a reasonable cost. A four-port USB 2.0 hub will cost about US$7 - $15, and don't bother with a USB 3.0 hub, as the Pi only has a USB 2.0 interface.

Mounting Drive Partitions

You can plug in a USB flash or hard disk drive while the Pi is running without any problem. The USB device will be sensed by the Pi, however, it probably won't be added to the operating system's file system. That will require some manual intervention to "mount" the device onto the file system. A flash or hard drive may be configured with one, or more, partitions, which you can create yourself in the unlikely event that the drive doesn't already have any. Creating partitions is covered in a separate tutorial: [TBD]

USB Drive Hierarchy

Linux hardware devices are organized and identified by letters under the file system's device hierarchy, /dev. USB drives attached to the Pi are found under the file system as /dev/sdX, where "X" starts with the letter "a" representing the first USB drive, "b" for the second drive, "c" for the third, etc. The individual partitions on each drive are represented by incremented numbers, starting with "1". So, the first partition on the first USB drive is located in the file system as /dev/sda1, the second partition on that drive is /dev/sda2, the third partition on a third drive would be /dev/sdc3, etc.

Mount Points

In order to make a partition accessible to the file system, you need to use the Linux "mount" command, referencing the hardware (e.g., /dev/hda1) and a "mount point" in the file system, which is any empty directory, usually created for the purpose by the user. There is a canonical (i.e., standard) directory path where mount points are usually created: the /mnt directory. You can create a new directory under /mnt for each partition to be mounted, and it can be named anything you want as long as it doesn't contain any spaces. You might want to name it to correspond to the drive's physical characteristics, e.g., /mnt/1GB_USB_flash, or /mnt/120GB_USB_hard_disk.

Mounting a Partition

After creating the mount point for a partition, the only thing left to do is to actually mount the drive partition, e.g.:

sudo mount -o uid=pi,gid=pi /dev/sda1 /mnt/1GB_USB_flash

where:

-o (lowercase letter "o", not the number zero) specifies that an option string follows

uid=pi,gid=pi specifies the user ID is user "pi" and the group ID is "pi" (note there are no spaces allowed between these terms)

/dev/sda1 is the first partition on the first USB drive

/mnt/1GB_USB_flash is the mount point directory

Mounting a Partition at Boot

To automatically mount a partition at boot time, the /etc/fstab file must be edited, and a line added for each partition and mountpoint:

 neil@raspbian:~$ cat /etc/fstab
 proc            /proc           proc    defaults        0       0
 /dev/mmcblk0p1  /boot           vfat    defaults        0       0
 #/dev/mmcblk0p3  none            swap    sw              0       0
 /dev/mmcblk0p4	/mnt/sdcard	ext4	rw,defaults	0	0
 /dev/sda1	/mnt/usbdisk	ext3	rw,defaults	0	0
 /dev/sda3	none		swap	sw		0	0

In this example, we are mounting one partition for read-write access at /mnt/usbdisk, and also moving the swap file from SD card to USB. The # character in the first column means that the rest of that line is treated as a comment. For more information see man fstab(5).

Operations on a Mounted Partition

If all goes well, you will see nothing after pressing the Return key other than another command line prompt, which means there were no errors encountered during the mount process. You can then access the partition by referring it via /mnt/1GB_USB_flash/... where the "..." would be the file system path to the directories and files on which you want to perform an operation. For example, to list the details about the files at the top of the partition, you would simply type:

ls -l /mnt/1GB_USB_flash

To copy a file named "file1.txt" to another file named "file2.txt" in the directory "my_directory", you would type:

cp /mnt/1GB_USB_flash/my_directory/file.txt /mnt/1GB_USB_flash/my_directory/file2.txt

Of course, you can make this a lot simpler by just changing the current directory to my_directory like this:

cd /mnt/1GB_USB_flash/my_directory

To run an executable file "my_program.bin" while in that directory, you would type:

./my_program.bin

You can copy or move files and directories to or from any directory in the mounted partition by just referring to the appropriate path to that directory in the partition.

Unmounting a Partition Before Drive Disconnection

When you're done with a drive and want to disconnect it from the Pi, you should unmount the drive by using the "umount" command (notice there is no letter "n" between the "u" and the "m") and referring to the mount point:

sudo umount /mnt/1GB_USB_flash

Other mount and umount Features

You can learn more about mount and umount features by typing mount -h or mount --help, umount -h or umount --help, or via their "man" (manual) pages by typing man mount or man umount.
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