I have read this entire thread and will post my issues in hopes to get more information. I am going to try the HD Cable and will report my results.
Last week, the beach ball started making an appearance a few time and it started getting slower and slower.
I did PRAM, SMC, NVRAM, ETC.... Ran Disk Utility, EtreCheck and then Reinstalled....and it got slower each time, even took me 8 hours in do the reInstall.
I figured the worst case and it must be the Hard Drive. Drove to OfficeMax and got a 1TB drive, and then, it wouldn't let the Disk Utility reformat to Mac OS.
Back to the store, spent 2 hours with the guy who had Max experience and we couldn't get anything to work, ever tried reformat in a PC and NOTHING.
Exchanged drive, and it worked....whew, finally. But Wait, there's more.
Got home, started installing my stuff while surfing the web and chatching up on email and BAM, i couldn't get on any Apple website or iCloud, it was like the internet was down, but other site worked fine - and Mail kept crashing.
REBOOT... everything seemed OK for an hour or two, then iPhoto started giving me an error message and then wouldnt reopen.
REBOOT...this time i get a Warning pop up that said: Unapproved Caller SecurityAgent may be invoked by Apple Software.
WHAT???
As of now, the computer will run for an hour or two and then it starts acting up...
WHAT IN THE MAC UNIVERSE CAN BE WRONG????
Anybody throw me an idea or some way to trouble-shoot this, would REALLY appreciate it!