Hello, yes that is very strange, but boot from your Install Disc & see if it shows up in DU.
"Try Disk Utility
1. Insert the Mac OS X Install disc, then restart the computer while holding the C key.
2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu at the top of the screen. (In Mac OS X 10.4 or later, you must select your language first.)
Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.
3. Click the First Aid tab.
4. Select your Mac OS X volume.
5. Click Repair. Disk Utility checks and repairs the disk."
As I wrote before...
NO disk is reconized by OS X. After OS X started, Disk Utils, or even System Information reconizes no external USB disk connected. However, they are all reconized before OS X startup.
And all these USB disks are OK and reconized in any other computer (Macs and PCs).
Might be time to replace the PRAM Battery, 4 years is close to their lifespan, far less if ever without AC power, & can cause strange startup problems...
No solution yet. PRAM batrery appears to be OK. And if I connect any USB device (mouse, keyboard, etc) they work fine, except any kind of USB disk (pendrive, external HD, etc). I did already reinstall OS X sometimes and did not solve.