Interesting message when linking a Group to a new RVT file… basically, if you have a Model Group in your project that has Excluded Elements (either deliberately or due to some unavoidable issue) and you then Link that Group, the excluded elements will be restored in the Link.
Excluded elements are also restored for Attached Detail Groups if you choose to link them as views, but you won’t get the nice little message.
This could be one way of recovering the excluded data without having to create a new instance or use the ‘Restore All Excluded’ command on an existing instance…
There is a nasty little bug that sometimes causes Backups (and any attempted Restore) to fail on a workshared project. The way to avoid this problem is so simple that it should probably be implemented on all workshared projects in your office:
check Regenerate if view/sheet is not up-to-date. This will create a thumbnail for the non-opened view to be included with the initial backup version and eliminate the error message when saving the backup version.
Cannot Restore Backup Version an Unknown Error Occurred – The Revit Clinic
However – if you have already come across this problem and it is too late to check the above box … you may be able to get the Restore to work IF you reinstate an appropriately named preview.xxxx.dat file into the _backup folder. Its worth a try if you are struggling to recover some important data.