Thank you for your feedback wrisco,
Everything 1.5.0.1297a adds support for finding duplicates of multiple properties with the
dupe: search function.
For example:
dupe:size;date-modified
-or-
dupe:size;dm
Another way I could imagine this to work would be some kind of custom columns, e.g. a single "modified date, filesize" column, then check this column for duplicates.
Maybe this could be done with regex capture groups.
I will look into this..
Since there is a "Full Path UTF-8 Byte Length" column, I hope it's not unreasonable to suggest a full path column without the drive letter (and maybe a drive letter column)..?

That would allow me to find the files I haven't copied to my mirror-drive.
Please try using the Regular Expression Match 1 property and including the following in your search:
Right click the Regular Expression Match 1 column header and right-click Find duplicates... and click Find Unique (not duplicated)...
-or-
If your mirror-drive is included in your Everything, try the following search:
regex:path:c:(.*)$ !fileexists:d:\1
where D: is your mirrored drive.
file-exists: