Thanks everyone, for taking the trouble to reply!

No, I don't want to try to include error-correction too. In the end, usable and useful error correction takes as much space as multiple copies of the files do. [I keep one master collection and two full copies on different discs.] But I would like to be able to detect file corruption before I back up the corrupted data on top of the good! [It was doing that that stimulated my interest in checksumming and file verification.]

One detail of the implementation intrigues me, though. Some files in Windows seem to have tags, reflecting sampling rate or composer for music files, and so on. I am wondering how these tags can be written and read programmatically under Windows? My thought is that I could use a tag to store the checksum for the file. This would be very handy, save loads of files containing the checksums, and make it easier to associate each file with its own checksum. This will, in turn, simplify the detection of new (un-checksummed) files in a folder.

So, a 'checksum' tag for each file? How is it done, does anyone know? TIA!