TL;DR: If you have problem using Ubuntu 11.04 from USB, remove the U3 launchpad if you do not need it at all. It may solve your problem.
I am not a Ubuntu/Linux user but I do keep a copy of Ubuntu LiveUSB in my Sandisk cruzer micro 8GB USB stick in case of system failure. It comes with the U3 lanuchpad that I never use.
The USB stick was working happily with Ubuntu 10.04 and 10.10. However when I formatted the stick and put Ubuntu 11.04 (checksum is ok) into it, it failed to boot from the stick every single time. I was getting this error even though I have disabled all(one) my physical DVD-RWs in BIOS:
The error message appeared when I saw the Ubuntu logo and the five animating dots. So I decided to remove U3 launchpad and voilà! I could boot for the first time (and 6th time in a row without problem now).
mounting /dev/sr0 on /cdrom. failed Device or resource busy.
(I do not remember the wording...)
sidenote: My physical DVD-RW drive is still blocking the bootup half of the times even if there is no discs in it. Disabling the drive in BIOS solve the problem. The error occurs before I see the five animating dots. The screen stops at
[figure varying each time] scsi 0:0:0:0: CD-ROM PIONEER DVD-RW DVR-111 1.29 PQ:0 ANSI:5