Les Mikesell wrote:
USB disks _can_ have partitions (obviously, since you can stick about any drive into a usb adapter), but small ones typically don't and you don't need them to boot. The bootdisk.img layout appears to be a vfat on the raw disk (no partitioning) with syslinux configure to make it boot.
The instructions in <http://docs.redhat.com/docs/en- US/Red_Hat_Enterprise_Linux/5/html/Installation_Guide/ certainly advise making a partition /dev/sdb1 with partition type b and running mkdosfs on it.
I didn't actually run syslinux after dd-ing; the CentOS instructions don't say you should. But I'll try it later, though I now have a reliable if lengthy way of installing CentOS on a machine without a CD drive, by following the instructions in the redhat document above (with one slight change I mentioned earlier).