Using a different Mac, Carbon Copy Cloner also failed to clone my computer's drive (in target disk mode) onto another drive, saying that "an error occurred while trying to access /.symAVx86QSFile, which although I have no idea what that is, it sounds consistent with your suspicion. Yes, it's an old computer, exactly nine years old. Thank you so much for this very helpful information. Do you know the approximate age of that hard drive? On a machine old enough to run Snow Leopard, that's a distinct possibility.
is that correct? If so, my concern is that this is more likely to be caused by something like system or disk corruption on the boot drive, which would be very concerning given that your existing backup drive just failed. If I'm understanding correctly, you're having system crashes when trying to make backups with Carbon Copy Cloner. To be honest, malware would be one of the last things I'd think of in such a case. However, it's important to note that system crashes are not a typical symptom of malware on the Mac. So there wouldn't be any way to scan that with Malwarebytes.
HOW TO REMOVE MALWARE FROM MAC BOOK 10.6 OLDER VERSION FOR MAC
Unfortunately, no, there is no version of Malwarebytes for Mac capable of running on Mac OS X 10.6, and all versions only scan the current boot drive.