Pretty much as stated above, nothing happens to the files. I personally tested enabling deduplication in 1809 and upgraded to 1903, installed the 18362 dedup pack and was back up and running. The same will apply going from 1903/9 to 2004. As far as trying to access a data deduplicated volume on a machine that doesn't have that feature, the host simply will not be able to access the files on that volume... well that's technically not entirely accurate. Any files smaller than 32kb would be able to be accessed as that is the minimum file size cuttoff for dedup. If you REALLY can't wait to hop over to 2004 and a VM or WTG installation just doesn't cut it for you. Best practice would be to disable deduplication of whatever volumes you need to access BEFORE upgrading or clean installing. You can do that by using the start-dedupjob command with the unoptomization job type. Code: Start-DedupJob -Volume "D:" -Type Unoptimization -Full -Wait The above would unoptomize volume "D:" the full switch probably isn't really needed and the -Wait switch will make powershell show a progress bar in the terminal.