Try this; Shut off the RAID Array, unhook the 2nd. HDD in the array, should be on IDE RAID channel #4, leave the HDD hooked up to IDE RAID channel #3, or transfer that drive to IDE channel #1 or #2, then install the other drive (the one you wish to transfer data from), as a slave to that single "master" drive, transfer the info to the master, then remove the slave and set everything back up as it was, set the array back to the original settings and rebuild & synchronize the array. Or you need to add 2 slave drives of the same exact type & size to the drives existing on the array (1 to each). That is actually the problem you face on boot up...The slave increases the size of 1 of the drives in the array...Size imbalance, plain & simple. And IDE RAID depends on that balance being there or it needs to truncate the larger of the 2 drives in the array.
I am assuming in stating the above method that you are using a mirrored array in the first place, and that you have on-board RAID w/ 4 IDE channels, 2 regular IDE, and 2 IDE RAID channels.
Remember that knowledge of the whole problem situation is really king! If I assumed anything incorrectly, you need to give more info please.
Or, you could do like I do and either set up the drive in another box, zip or burn to CD the data off you want to transfer and load it onto the RAID unit drive properly.
