Windows 7 - Bitlocker acts different on laptops and desktops

Our firm is in the process of rolling out Windows 7 64-bit and we've found that our desktop users are locking up their machines if they have a USB flash drive plugged in while booting up. Obviously this is causing bitlocker to come up. The issue is, that this is not happening on our laptops. Can anyone shed some light on this. Is there a setting we're missing or does it just work differently on laptops.

I verified the primary boot device is the HDD for the laptop and desktop but I don't know what else to check. Let me know if any more information is needed.

What hardware (mfr, models, year, etc) are the desktops and laptops?

All Lenovo machines. The laptops are T400's, T410's and T420's. The desktop model is MT-M 5536. The machines are all less than a year old. I don't have more specifics on the desktops handy.

Nobody's had any similar issues?

Are they all on SP-1?

Network setup?

Yes, all on SP-1. Not sure what info you're looking for Network setup.

Im clueless just trying to find the similarities and differences.

Well they're both on a wired connection and have the same image (aside from the drivers of course). Can't think of anything else that might help.