using dd to duplicate disks/partitions of slightly different sizes - works?

Steve Franks stevefranks at
Sat Dec 1 09:29:16 PST 2007

I've got two 160GB disks.  Different manufacturers, so different #
bytes.  One is FreeBSD, the other blank.  I'd like to backup my system
for the fastest possible recovery after a crash - move the plug and
power up.  I have gmirrored before, but I just wanted to do a quick
dd, since I don't want to abuse my cheapo powersupply (has 4 disks
already on it).

Two questions:

(1) If I dd from the smaller to the larger, will it work?  What
happens to the extra, say 5MB of unused space - will my partition info
be messed up?

(2) If I dd from the larger to the smaller (df reports only 50% used
anyway) is there a way to make sure there is no info in the 5MB at the
end that will overflow the smaller, and again, will my partitions be

The other option is just to fdisk & label the other disk, then rsync
everything to it.  Is that the wiser choice?


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