Thanks for responding Mark.
I did it this way,
The microSd with the boot partition is still there on the beagle board.
I have made no modifications to the u-boot, kernel or root file system.
I am using the files provided in BeagleBoard-xM xMTEST_Beta_4_25 at Circuitco Support Wiki.
its just that I have a USB drive with two partitions sda1 and sda2. of which sda1 has the
root file system(same as the microSD partition2) and sda2 is ext3 formatted free space.
so when the device starts up the kernel is loaded from the microSD partition1 and
with the new lines added in the uENV.txt in the boot partition of microSD, I was expecting the rootfilesystem to be loaded form usb, ie. from /dev/sda1.
somehow it is not happening as I expected. Is there something totally wrong in what I did…