The iMac computer in question was slowly losing its performance. The hard drive was determined to be failing, so I had to replace it.
Apart from one dropped screw, the repair went well. iFixit had just what I needed in terms of tools and advice. It took longer to remove data from the hard drive than it did to fix the machine. Luckily, everything was recoverable from backups or the drive itself.
A lightly magnetized screwdriver is a huge boon when working in these machines, as it will keep the screws from venturing into places they should not!