I've used unetbootin in the past just fine however recently I was trying to install on a really old PC for use as a server and it would hang at the USB boot menu when I tried to boot over USB.

I managed to install Debian successfully using this guide which I've duplicated in my answer

The idea is the following:

1. Download the boot.img.gz for the relevant version of Debian from here. You will need to change the version to make it match the ISO that you have. http://ftp.debian.org/debian/dists/Debian6.0.2/main/installer-amd64/current/images/hd-media/

2. Unmount the target USB disk

3. run gzip -dc boot.img.gz > /dev/disk# to your respective usb

4. After it finishes (it took an oddly long time for a 50MB image, compression I guess), disconnect and reconnect the drive. Simply drag and drop the ISO into the root of the newly created partition.

Try and install! Worked for my Dell Optiplex 380

I've used unetbootin in the past just fine however recently I was trying to install on a really old PC for use as a server and it would hang at the USB boot menu when I tried to boot over USB.

I managed to install Debian successfully using this guide which I've duplicated in my answer

The idea is the following:

1. Download the boot.img.gz for the relevant version of Debian from here. You will need to change the version to make it match the ISO that you have. http://ftp.debian.org/debian/dists/stable/main/installer-amd64/current/images/hd-media/

2. Unmount the target USB disk

3. run gzip -dc boot.img.gz > /dev/disk# to your respective usb

4. After it finishes (it took an oddly long time for a 50MB image, compression I guess), disconnect and reconnect the drive. Simply drag and drop the ISO into the root of the newly created partition.

Try and install! Worked for my Dell Optiplex 380