Endless boot loop after RAM install
I'm trying to help my sister-in-law by upgrading RAM and adding an SSD to her late 2009 21.5" iMac. I have successfully managed this in my own machine and created a Fusion Drive.
I installed 4x4gb RAM that match specs of machine and then installed Sierra to the computer. I ran into problems booting the machine after that (never successfully made it into Sierra). Now, the machine boots endlessly, progress bar makes it about 3/4 of the way through then it reboots.
I have (in no particular order):
Put the original 2x2gb RAM back in: no change.
Booted to Recovery mode: reboots before it gets to recovery options.
Booted to Internet Recovery: reboots at Apple logo after getting past spinning globe.
Booted to external drive with a bootable Sierra installer on it (both an external HD and an SD card): crashes and reboots at same spot as before.
Booted in safe mode: get error message and reboot.
Booted to my 27" iMac running Sierra as the Target Disk: kernel panic and reboot (have error message if important)
Looking for any help of where to go from here. All I want is a clean install on the machine with the SSD in place of the optical drive (using the iFixit mount). At this point, I'd even settle for just the SSD if need be, without the Fusion Drive I'd planned. Thanks in advance for any info any of you can help with.