How do I setup unison to sync a folder one way

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http://serverfault.com – I have a 1tb NAS that has a 1tb usb external hard attached I have prepared the file system on the usb disk and mounted it I want to 100% sync my data from my nas to the usb disk - but I want it to be incremental and only have the NAS as the 'master' - eg if a file changes on the usb external hard drive I want it to ignore this change as its not the live version (not that I think the files will change on the usb disk but im paranoid the live could get overwritten) Also if a file gets deleted on live I want to retain the deleted file on the usb disk Can unison sync one-way and achive the abo (HowTos)