Did you use the
Rename, Move, and Copy Files function? You should have, and then any Sidecar files would have been moved with the files, and your tags would still be there.
There are some caveats. Read the Wiki article carefully. You may need to move the Sidecar files manually now if you used an external application to move the files.
1. I think you are obsessing over the "Read-Only" status of files a bit, but yes, you need write access to be able to write tags into files. In a normal windows environment if you are copying files to disk, you should have access to read and write to them. They shouldn't be copied to you computer in "Read-Only" status, but that will depend on the source. If they are, change the status.
You will have external Sidecar XML files if you have set the "Store tags in external sidecar files if the file type does not support tagging: Video" in the Options/General, and the file type can't store tags, or MC can't write the tags to the files.
If you lost the Sidecar XML tag files, you could run "Update Tags (From Library)" to update or recreate the sidecar files. But MC should already know about the tags, particularly if they were written to the files. For most audio file formats, tags will be written into the files. For most video formats I'm not sure they are. See below.
2. Always use the "Rename, Move, and Copy Files" function in future. Read the Wiki carefully. Do not leave the "Move all files in folder" flag set unless you really understand what it means, and need to do that. Now that you have moved the files, you may be able to find the Sidecar XML files that have all your tags in them and move them to the same location as the files you moved, and get all your tags back.
If you moved the files outside MC, you will have needed to run, or have running in the background, Auto-Import with the "Update for external changes" flag set. If you did move the files externally to MC, and let Auto-Import run without that flag, then changing the flag now and manually running Auto-Import may get all your tags back.
EDIT: Brian's suggestion may work as well, although if external changes were made, I'm not sure the RM&C tool is the best option. Auto-Import with the "Update for external changes" flag set would seem more logical, but it may be too late for that if you have already imported without the correct setting. Restoring an earlier database and running Auto-Import with the "Update for external changes" flag set would definitely recover all tags for files up to the time of the backup. You could try Auto-Import without a restore first, no harm in that. If you didn't get the tags back, restore a backup, set that flag again, and run Auto-Import.3. I don't know why you are getting that error, unless you haven't set the "Store tags in external sidecar files if the file type does not support tagging: Video" in the Options/General, and MC is unable to save the tags to the files. I don't get the errors, but I have that setting.
Test using only a few files of each type, and use Undo if you need to. Do not select every file of a particular type unless you understand
exactly what is going to happen. In fact, it is never a good idea to select a large number of files to make changes all at once, at least until you have tested your settings properly with a few files.
4. No. Okay, some people like to use certain other programs to change tags. But if you do that, you need to understand the consequences for MC, and at a minimum you will need to run "Update Library from Tags", and if you do that, you had better be very very sure that you have all the tag information you want in the files, ready to update the library.
If you take a look at the attached image, see where the text "AVI - 40:43 - 350 MB" is, when the tag Action Window is open? Click on the equivalent text for your file, and the Action Window will change to display the tags actually stored in the file, rather than in a Sidecar XML file. I only have a few WMV files, and they all have tags in them. None of my other video files have tags in them. I have set in Options/General both the "Update tags when file info changes" and the "Store tags in external sidecar files if the file type does not support tagging: Video" flags.
I should look into why tags don't seem to be stored in most Video files, because I thought they should be for many formats. But for now I rely on the external sidecar XML files to store tags for Video.