In case anyone else runs into the same quirk:
A strange glitch in Old Reliable, solved.
Recorded a show. Made track marks during some applause. Named it and tagged it (Mp3tag) to put on another player. Then I thought I'd edit some of those track marks on the PCM-M10.
Sorry, no: "Invalid Operation."
The manual's explanation of the error message was irrelevant.
• You cannot copy a track or divide a track, because there is more
than one folder that has files with same name. Change the file
names.
• You cannot divide or set a track mark to a track not recorded using
the PCM recorder.
• The “Divide All T-MARK(Track Marks)” cannot be executed
because there is a track mark within 0.5 seconds from the dividing
position.
• The file name has reached the maximum number of characters;
you cannot divide or add “TAKE/KEEP.” Shorten the file name.
I moved it to a different folder, took off the (short) artist name I had added to the filename. Still Invalid.
But wait a minute...."not recorded using the PCM recorder." Why would it think that?
The only thing changed in the file was the tag. Took it out, put back My Recording and 150109_04 as title. Fixed. FYI.