Turns out on Jaunty I had 2.6.31-6.25, but the USB drive had 2.6.31-6.26. When I downgraded to
2.6.31-6.25~radeon2 on the USB drive it worked. So the problem seems to have been introduced between the 2.6.31-6.25 and 2.6.31-6.26 Ubuntu kernel, and between 2.6.31-rc6 and 2.6.31-rc7 mainline kernels.
From the changelog, 2.6.31-6.26 seems to only bring package (abstracted debian) and ports changes...
Turns out on Jaunty I had 2.6.31-6.25, but the USB drive had 2.6.31-6.26. When I downgraded to
2.6.31-6.25~radeon2 on the USB drive it worked. So the problem seems to have been introduced between the 2.6.31-6.25 and 2.6.31-6.26 Ubuntu kernel, and between 2.6.31-rc6 and 2.6.31-rc7 mainline kernels.
From the changelog, 2.6.31-6.26 seems to only bring package (abstracted debian) and ports changes...