I'm following this: http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-ubuntu
and I know a lot of boot-disk creation stuff will wipe my USB---it's 30GB which is more than I can comfortably back up to my HD right now, but I would like to install ubuntu on a friend's machine.
These instructions say: "If you’re not yet sure about installing Ubuntu, you can try it out without affecting your current system." So, that suggests there won't be any deleting, but I'm hesitant to push the "Make startup disk" button here:
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...when there's a button so nearby called "Erase Disk" so close by---I would never want erasing my disk to be an option if it weren't necessary. Can someone confirm for me for sure whether this operation will delete my USB contents?
Creating an Ubuntu boot disk on your USB stick will wipe the contents of such USB stick, as far as I know. I'd recoomend to back up your data. But, why use a 30GB disk for that task? a 4GB USB stick will do the work and is quite inexpensive.
Well, I decided to chance it (I have no capacity to make backups, but needed the install disk).
Clicking "make startup disk" without erasing the disk produced the un-informative error "installation failed." I assume it might work if I erased the disk, but don't know for sure.
Anyhow, I can provide an answer to my own question---no, clicking on that button will not in itself format your disk. BUT you may need to format your disk to make any use of this program.
As far as I know, the "Make Startup Disk" option doesn't erase the disk, "Erase disk" will. When you create a bootable disk, you only append files to your USB device, not overwrite the contents.
If you wish to check this, try it on a USB device containing not-so-important data.
Almost all my USB data drives also contain a live system, usually System Rescue CD. But I added a data folder AFTER creating the startup drives.
In theory there is no need to wipe the data: an install USB drive is just a file system + a file tree + boot code in the master boot record/sector of the drive. It can carry and keep all sorts of other data.
In practice, it all depends on the procedure. You could indeed manually transfer the content of the ISO + manually install grub.
I don't know anyone who does it with Ubuntu, and it seems to me that none of the previous answers refers to this.
This leaves using the graphical-interface startup creator tools.
With the standard Ubuntu tool I too always got errors trying to avoid blanking the drive. If you look for informal bug reports in forums you find a lot of them.
Alternative startup-disk makers like unetbootin seem to wipe the drive as well.