Thanks, Virtual for your thoughts and experience. It's good to know you and others from the late/lamented MFI forums are still available when I'm in a jam.
Since posting my query, I spoke with a tech at Apple Support, who suggested (a) that I check my Disk Utility for its current report re: SMART and (b) access the Apple Hardware Test on my system install disk and see if it reported any problem. Disk Utility indicated that SMART status was "verified" and AHT in an extended check reported "no trouble found." There was no indication that AHT included the temperature of the internal hard drive in its battery of tests. I also reviewed an extended forum discussion from earlier this year at the MicroMat TTP 5.0 user forum regarding SMART failure reports due to "temperature changes" (attribute 190), which suggested the problem is not uncommon (especially in Seagate hard drives, although my drive is an Nvidia), and the accuracy, meaning, and implications of some SMART reports are uncertain. Apparently not all applications check all SMART factors. MicroMat doubted if Disk Utility checks temperature changes, for instance. I wasn't able to find any relevant info on any of these matters through Nvidia's web site.
In the MM forum discussion there were some references to an application (widget?) which can report the current internal (drive? cpu?) temperature. Are you familiar with this and its utility in my case?
BTW, the Apple tech suggested I had little to worry about due to drive failure as a result of persistent high temperatures because (a) the system would react with shutting down and a warning before meltdown; and (b) the one year warranty on my iMac would replace the defective drive in such an event.
