I purchased a refurbished 4th generation HP branded Apple iPod for my friend today. When I was setting it up for him I noticed it was formatted for Windows and he uses a Mac, like me. To change it to Mac-format, so he could install firmware updates, I restored the iPod in iTunes. I've done this a number of times with my Mac and my iPod Mini, however with a firewire cable. I didn't know that if you restore this generation iPod with a USB connection, you need to connect it to a wall charger afterwards to complete the firmware update.
HP did not ship this iPod with a firewire cable or a power adapter. It was asking to be connected to a wall adapter but we did not have one. We bought a car power adapter, but this didn't work. We tried inserting the USB cable half-way, but that did not work either. What finally worked was taking the iPod to a Radio Shack and plugging it into an iPod clock radio/boombox type device; the firmware installed, the iPod rebooted, and we have a Mac-formatted iPod working perfectly now.
No amount of resetting or attempts to put the iPod in disk mode prior to this would work. Hope this helps someone.