How should I format a USB prior to writing an ISO to it?

Question

I found these Debian EeePC HowToInstallUsingStandardInstaller instructions at the Debian Wiki, stating that I can install an ISO to a USB using this:

dd if=debian-*-netinst.iso of=/dev/sdX

• Do I need to delete the contents of the USB prior to running this so that nothing is left at the end of the USB?
• Does it matter what filesystem is already present on the USB prior to running this command?

Answer 1

Writing to the device (/dev/sdX) instead of to a file system (/mount/point) deletes the file system on the device anyway so there's no need for mkfs before.

There is also no need to erase data in the non-written area. It is not visible anyway. Linux file systems can without problems be smaller than the block device they live on.

Answer 2

The dd command overwrites everything on the USB device. No preparaton is required.

Answer 3

You don't need to erase the old data first. But you can if you want to. Sometimes it's a good idea. Particularly if you're giving it to someone.

This should do the trick:

Linux:

dd if=/dev/zero of=/dev/sdx

Macintosh:

dd if=/dev/zero of=/dev/diskx

diskutil zeroDisk /dev/diskx

Where sdx or diskx is the target disk. Usually, sda/disk0 is the first disk, containing the main OS, etc. (i.e. SCSI/SATA Disk A), followed by sdb/disk1, sdc/disk2, and so on.
If you have one internal disk (/dev/sda or /dev/disk0), and you connect an external drive via USB; the external drive will be /dev/sdb or /dev/disk1. You get the idea. SD cards are designated names like /dev/mmcblk0 and so on, in a similar fashion.

Some refer to this as zeroing or low-level formatting. The pseudo-file /dev/zero can be thought of as being similar to /dev/null; but designed to be read from, rather than written to. It will supply a steady stream of ASCII NUL (0x00) bytes to whatever process tries to read from it.

The original filesystem will be overwritten.

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Some additional options that people commonly like to use with dd, to manipulate block size & data read/write speeds follow the syntax:

bs=512kB count=1 

Also, I personally like to have a running status/progress report so I can tell what's going on; to do this, just add the following to the tail end of the command:

status=progress

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Take care to specify the correct disk. Especially if copying/pasting commands like this directly into a shell. It takes milliseconds to bork the wrong partition table. To list your storage devices, begin with:

Linux:

lsblk

Macintosh:

diskutil list

The output will look something like this:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            499.4 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3