I have an older iMac (2008) that was running pretty well on El Capitan until a few months ago. It began to run incredibly slow. I mean 20 minutes to boot up and 2-5 minutes to open anything. I ran verify on disk and it had several issues but couldn't repair because it was the boot disk without a restore. I loaded El Capitan on a USB drive and booted from that. when I booted from the USB drive the system ran like new. In fact, I think it ran faster than when the iMac was new. I was now able to use the disk utility to repair the internal drive. It reported that it was all repaired and I completely reformatted the internal drive and attempted to install a fresh El Capitan. However even the install took forever (6 hours) and everything still runs at a crawl. Does the fact that it runs great using external and still crawls on internal point to a bad drive or am I overlooking another possibility? Thanks in advance for any suggestions.