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The reason why 'copy from source' isn't available is because you are transcoding to 32 bit float by the way - because your destination session is a different word length than your source.

The offset works - it's just not very intuitive. I have had a dialog with AVID about this. When there is a tempo map and a 'sequence start trigger' i.e. a bar 1 and tempo marker - it is a bit kafacta.

There is a setting in the top right that will 'maintain absolute timecode' or 'maintain relative timecode' - under that is a button (I believe it's under it - not in front of PT at the moment) that says something along the lines of 'offset session start time to match incoming session start time' and then the 'maintain absolute timecode' will no longer be gray.

In Pro Tools 12.5.1 the start time of current session appears as default - which is wrong as it then offsets incoming by that amount - so you have to click int that and make it zero - again NOT very intuitive and same on AVID for messing this up after so many versions.