I've searched the forums and can't seem to find anyone with this specific issue. I have two different Dell laptops, an XPS M1330 and an Inspiron e1705. I have an OS installed on a full size Seagate Freeagent 320GB external drive and it runs swimmingly. My BIOS detects it on both systems and boots straight away.
In an effort to have a more portable version (This drive is rather large, and has a huge base and a power supply) I bought a 500gb external Simpletech portable drive. Neither BIOS can see this drive. It doesn't have a power supply, but does have a Y connector so two USB ports can provide power. When I plug the drive in while in windows or OSX, it recognizes the drive straight away, as does Disk Manager in windows. The drive works fine.
I had this same problem with a 16GB Patriot USB stick I bought with the same purpose, neither computer would recognize it as a USB boot device.
Again, I can boot from USB without issue using other devices, just not these.
Is this a USB power issue? Dell "Premium" support provides no help here, they say "interesting" and that's about it.
An additional bit of info, the USB drive in question can boot if used with my desktop. Another sign that points directly to a problem with BOTH of these Dells. I'm really leaning towards this being a low power issue as both usb devices that don't work are low power type devices, no big power brick.