since I don't have an extra drive similar in size to my mythbox to use the dd command. Can I install knoppmyth from scratch on to a spare drive then reinstall the original drive. Then run a crop job to copy the files that have changed thereby mirroring the original drive?

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AMD Athlon 666mghz
asus a7a266 motherboard
1gig ram
160 gig drive
pvr 250
nvidia gforce 4 mx

Yes abast62, that'll work fine. You don't even need to set everything up on the spare drive, just get the partitions on the disk and get it formatted. Rsync will take care of making the content of the drives identical.