I use default files naming, directories, cover art locations etc. So why isn't the cover art copied over as part of the upgrade process?
Because it doesn't. Simple as that. There may be some logic to why the feature hasn't been added. Perhaps if it was, and you reinstalled MC21 a second time, it would copy the Cover Art over again, and maybe overwrite improved Cover Art files? For some people, copying Cover Art over may take a very long time, if they have lots of high quality images. However moving the images to a new directory on the same disk drive would be a very quick operation. But moving the files, or the directory, could be a very high risk thing to do automatically, particularly during a reinstallation. What about people who downgrade? Basically, Cover Art not stored beside the media file is something that is probably best handled manually.
If you want Cover Art to be carried forward through successive major upgrades of MC, just move it all to a location that does not sit below a MC version specific directory. Some people just put it in a location like "[Data Drive]:\JRiver\Cover Art" and never have to think about it again. That is the flexibility of MC, and allows you to totally manage your Cover Art.
MC21 did seeem to be finding cover art for some seasons/series, but it was doing this very very slowly. By slowly, I mean maybe one or two series PER DAY! This is on a machine that does nothing else, and runs 24 hours a day. What is triggering it to find data for a series/season.
Maybe you were changing something for those Seasons/Series, such as doing some tagging, which changed the Date Modified of the media files, and hence triggered Auto-Import on the files, which then did the Get from Internet lookup for Cover Art, since it didn't exist.
Of course if what you were seeing was thumbnails rather than full Cover Art, that could also explain it. Particularly if you were seeing them on a MC Client rather than a MC Server. Have a read of this thread for some understanding:
http://yabb.jriver.com/interact/index.php?topic=104468.0Of course thumbnails are also stored under a MC version specific directory, in "AppData\Roaming\J River\Media Center 21\Thumbnails", but maybe those do get copied across. I didn't think so, but I'm not sure.
Is there a way to manually force it to look for missing cover art across a large set oif files.
I believe that if you Move or Copy the Cover Art files to their new location, as per the settings in the new version of MC, then select the files you want to fix, using any view or search result you want to, then Right-Click on one of the selected files, and navigate to ">Cover Art>Quick Find In File / Cover Art Directory" then all your Cover Art will be restored. However, depending on other factors, as above, the Thumbnails for the Cover Art may no longer exist. So also run the ">Cover Art>Rebuild Thumbnail" if that is the case.
BTW, I'm not positive, but I think when I do a major upgrade, for example from MC20 to MC21, that the "Options>File Location>Cover Art>Folder" was still pointing to the previous version of MC, i.e. MC20, until I went and manually changed it. Because that setting is stored in the database, I think. Even if not in the database, but in the Windows Registry, if I do a MC20 backup, major upgrade to MC21, restore the backup including settings, then that setting will come across, and still point to the MC20 location. Did you manually change that setting after the upgrade?