I opened up Activity Monitor to see if it was a RAM shortage when I noticed my CPU history was almost maxed out, but NOT by the photoshop process. My cpu cycles were getting chewed up by a system process called ATSServer .
My hard disk activity was high considering I wasn’t actively doing anything at the time.
A little research on the web indicated that ATSServer was OS X Spotlight’s indexing process. Others having this problem had recently copied a large number of PDF’s to their machine and that’s exactly what I had done too.
Spotlight doesn’t just index the filenames, where possible it indexes the entire contents of PDFs, emails, etc and the process seemed to be stuck trying to index the PDF’s I had recently added.
Activity Monitor showed that the process had a full 24hrs+ of cpu time so I don’t think it was simply a case of waiting longer for it to complete. Instead I went into Preferences->Spotlight and added the folder that had all the newly added PDF’s to the privacy folder settings. This, however, did not stop the runaway process nor the thrashing of the hard drive, so i took the decision to Force Quit the ATSServer process and all is well again.
I’m obviously going to have to reboot to start the service running and hopefully things will be back to normal.