I have 10.11.6 installed. Seemed a bit quirky so thought I would try the old standby of run the 10.11.6 Combo. Won't let me do it, says can't install on this disk, volume does not meet requirements. Went to App Store, Download not offered, clicked Install El Capitan - few seconds after clicking Install in the Installer get dio "Copy can't be Verified". Have not had trouble with Combos before and think an Install to a running system is an option. Been running the Mercury PCI eSATA for 18 months, updated Mavericks to El Cap on it. No sense to this idea but sometimes computers do things that make no sense. Did a SuperDuper to a clean HDD. Booted to the HDD and tried the Combo on my usual Boot SSD, nope. Tried the Combo update on the HDD, got the same Does Not Meet Requirements. OK. Erased the HDD and tried Install from the Apps store on HDD and got the same Can't Be Verified. Ideas?