That didn't work. It didn't work when I tried it before, but I tried it again, and it still didn't work; however, I did manage to switch partitons. I did it by going to the Apple icon in the menu bar, then System Preferences, then
Startup Disc, then Snow Leopard. It was all worthwhile. I had stupidly erased part of a precious 20-year old cassette of mine while preparing to transfer it to my iTunes library. I had done it before but in the Snow Leopard environment and with an older version of iTunes, and It wouldn't play with Mountain Lion and the latest iTunes version. Anyway, it was fine where I recorded it, so when I have time I'll figure out to get it to play in Mountain Lion/current iTunes. Maybe burning a CD, then transferring it back to Mountain lion.... I'm sure that something can be done. Thanks for your input.