Could not open PlatformSupport.plist - OS X Yosemite does not boot

Hi there,

so out of pretty much nothing, my Finder stopped working today morning. I rebooted and went to work. When I came back, the iMac 2007 was booted, but the Finder did not respond at all, so I rebooted again. And again - and again, because nothing happened anymore. I once got the grey loading bar, but only once - after trying NVRAM-Reset and every possible boot-option, I even got the prohibition sign. Sometimes, my iMac reboots by itself after some time; if I start without pressing anything, the apple symbol doesn't even come up.

I tried booting from the install disc, it worked. I went on to disk utility to try to repair the disk (it said B-Tree ("B-Baum" in german) was broken), did not work. I formatted the disk and tried to recover from a (working) backup - it stopped after an hour saying it was not able to recover the backup.

After that, I tried the verbose mode again, and it suddenly worked showing this:

20 times this: "Could not open PlatformSupport.plist"

Must be out of unencrypted bootable images

Found 1 recovery sets that are supported by this board

efiboot loaded from device: Acpi (PMP0A03,0)/Pci(1D|0)/Usb(0, 0)/HD(Part3,SigC2321BE4-3204-4CDC-B046-8873A7485AFA)

efiboot file path: \Backups.backupdb\.RecoverySets\0\com.apple.recovery.boot\boot.efi

(Yes, I copied that by hand)

If I boot up in Single User mode, I get this:

20 times this: "Could not open PlatformSupport.plist"

Must be out of unencrypted bootable images

Found 1 recovery sets that are supported by this board

I can't enter anything, and again, after some time, it reboots.

I'm out of options, I've tried pretty much everything; everything I can think of, that is, despite carrying it to the Apple Store (that would be my last option, but I would prefer to fix this by myself, since this computer is pretty old already and letting Apple Support fix it may not be worth it after all).

Oh, system is 10.10.4, iMac 2007 20''. No, it is not possible to boot from a recovery drive. Nothing happens!

Anyone got any suggestions?

Greetings and thanks in advance,

jatenk