How to compare two hard drives (or folder) to find missing files in one of them?

Beyond Compare (Scooter Software) is very good software for this task (Drive and Folder compares) and is reasonably economical.

There are versions for Mac, Windows and Linux.

I use Beyond Compare to compare hard drives when I have copied A to B and made changes to one drive after the copy.

Beyond Compare Features

COMPARE FILES, FOLDERS

You can compare entire drives and folders at high speed, checking just sizes and modified times. Or, thoroughly verify every file with byte-by-byte comparisons. FTP sites, cloud storage, and zip files are integrated seamlessly, and powerful filters allow you to limit what you see to only what you're interested in.

Once you've found specific files you're interested in, Beyond Compare can intelligently pick the best way to compare and display them. Text files can be viewed and edited with syntax highlighting and comparison rules tweaked specifically for documents, source code, and HTML. The text contents of Microsoft Word .doc and Adobe .pdf files can also be compared but not edited. Data files, executables, binary data, and images all have dedicated viewers as well, so you always have a clear view of the changes.

Compare files

Side-by-side display

Color highlighting of differences within lines

Display filters to show only differences

Jump to next/previous difference within file or folder

Printed and HTML comparison reports

Unicode and MBCS support

Thumbnail overview of comparison

Can convert files before comparing them

Compare files or clipboard contents

Automatic backups when saving