System: 24 inch iMac (mid-2007) MA878LL Current configuration: This system has been working beautifully for the last 6 months since I upgraded the HDD with a 1TB SSD and installed 6GB RAM. I have enabled TRIM on the SSD and the system has been working better than it did when I bought it. What has changed: When installing the latest OS X Yosemite v10.10.5 and Security Update 2015-006, upon reboot the system started to load then after 20 seconds of progress it displays the prohibitory symbol. When starting in verbose mode system hangs at “Still waiting for root device”. Also, the system no longer has a chime on startup. Steps taken so far to restore system: I attempted the following startup options with a directly connected keyboard and the results follow each option: Start up in Safe Mode – Went straight to prohibitory symbol. Start up to Startup Manager – Went straight to prohibitory symbol. Start up from a bootable CD or USB thumb drive– Went straight to prohibitory symbol from CD and USB drive. Start up to Apple Diagnostics – Went straight to prohibitory symbol. Start up from OS X Recovery – Went straight to prohibitory symbol. Start up from OS X Recovery from USB drive in option mode– When booting with option key it shows my primary SSD and USB Drive. Tried booting from both only to end in the prohibitory symbol. Reset NVRAM – Went straight to prohibitory symbol, although this time the startup chime came back and has been working for every reboot since the reset. Reset the SMC - No effect, went straight to prohibitory symbol. Start up in single-user mode – Went straight to prohibitory symbol and displays “Still waiting for root device”. Start up in target disk mode using firewire – This works and I was able to access the SSD from my MacBook. I created an image backup of my SSD to another drive. I also ran diagnostics on the drive with disk utility which revealed the drive is in perfect condition. Start up in verbose mode – Went straight to prohibitory symbol after hanging at OSX Kernel debugger, displays “Still waiting for root device” and repeats every 60 seconds. Firmware restoration – Suspecting a firmware issue, I created a firmware restoration disk from my MacBook and followed the steps from the Apple support page to install. According to Apple, I was directed to hold the power button until I hear 3 beeps continuously, then insert the firmware restoration CD. When I hold down the power button I only get one long beep and then it tries to start up, only ending in the prohibitory symbol and doesn’t even try to read the firmware restoration CD. The very last operation I made was replace the logic board battery and repeating the NVRAM reset along with the rest of the previously mentioned startup options which resulted in the same results identified. I understand this system is 8 years old but it has been working so well I feel I need to at least try to keep it alive longer. The only thing I have not tried is reinstalling the OS from scratch, but if I can’t even boot into recovery mode or from CD/USB, I find it will be impossible to even get to that step. I suspect it is a hardware issue that may be preventing any further progress but I wanted to at least put it out to the larger community for help on this issue.