Is there a way to migrate existing preview files?

I installed nextcloud 16 on an old arm box with ubuntu 18.04 and used that for years. Because of the storage limitation of that box, I mount the nextcloud data directory on an external storage. For now there is more than 10GB preview-generated data on the external storage in the appdata_xxxx folder.

Recently I got a new device with x86 ubuntun 20.04 and I installed a new nextcloud 18 via docker on this device, then I tried to mount the data direcotry to the same external storage as above, but it seems the existing preview data is not used by nextcloud. I also tried to dump the database from the old box and import it into the new device, however this even crashed the whole nextcloud.

Can any body help or give some tip to me whether the existing preview data can be migrated? Thanks a lot.

I am afraid not

Just pregenerate new previews as per:

sudo -u www-data php occ preview:pre-generate

Or for docker could be:

docker exec --user www-data CONTAINER_ID php occ preview:pre-generate

Of course you need a Preview Generator App enabled.