Really depends on the specific RAID controller. *Most* of the time, when
you set up a RAID0 (mirror), it can be done w/ existing data since all the
setup does is define a SOURCE and TARGET, then initialize the array by
copying from SOURCE to TARGET. IOW, it's non-destructive to the SOURCE.
RAID0 (stripping) is more dubious. I've noticed on my Promise FastTrak100
TX2 PCI RAID Controller, for example, even existing data in this case is a
none issue (other controllers may be problematic). It just leaves it alone
upon setup of the stripped array. Of course, while this makes setup
convenient (no need to backup and copy back files afterwards), you also LOSE
the efficiencies of the stripped array (at least for those files) for which
you defined the stripped array in the first place! IOW, the files are not
written across BOTH drives, they reside on only the one HD. That's why,
even when RAID0 doesn't destroy existing data, it's probably more effective
if you backup, clear the HD, and copy the files back. Even cloning the HD
partitions is not that effective because an image copy will be sector based,
not file based.
That said, if you have an existing RAID0 array (stripped), and want to add
RAID1 (mirroring) of those HDs, you'll probably have no issue w/ existing
data. But if in doubt, back it up (which you should do anyway, imo). The
only issue might be is the RAID configuration requires RAID0+1 to be define
in one operation, vs. RAID0 followed by RAID1. I can't imagine the latter
being a problem w/ existing data.
But the mere fact you nor I know for SURE is why a backup is critical. I
personally would never risk it UNLESS I had done the operation previously
and knew for certain the consequences.
HTH
Jim
