Stuck between Snow Leopard and Yosemite install-no escape!

[Randalls]

A co-worker has a 17inch macbook pro model A1927. Please take this with a grain of salt as I hastily wrote down this tale of woe. They were going to upgrade to Lion due to new options on their updated iPhone that would not run in Snow Leopard.
The Apple site offered only Yosemite as an upgrade, Lion was grayed out (they were currently running Snow Leopard). After about 20 minutes the Yosemite installer gets to the end of the install and crashes (this has occurred several times. Crash log is 1k lines of errors. The Mac Pro will not boot now. They tried re-installing snow leopard off an install disc to go 'back' to a working OS with same crash log at end of install.

They do have a time machine back up but no way to boot off of an existing back up.

The S.O. has a macbook pro as well. Can they use this (via usb) to check the ailing mac HD and run disk utility?

Was the Yosemite install crashing because of firmware issues? Any thoughts on next step(s) appreciated.

So, how would they get it out of the crashing loop and booted up?
Should they forge ahead with the Yosemite install or do a nuke and pave and reinstall Snow Leopard and apply the time machine backup. All this is pretty new to me. Any help appreciated.

Edited to change O.S. to Yosemite upgrade. My bad.

[Paul F.]

My inclination would be to nuke and pave, install Snow Leopard, then IMMEDIATELY install Mavericks, THEN restore from Time Machine Backup.

[deckeda]

I'd boot from the DVD, erase the HD, install 10.6, log into the App Store, get Mavericks and save the installer to a thumb drive.

Then install Mavericks, then get whatever you want from Time Machine.

[deckeda]

Paul beat me to it. But yeah, get the OS squared away before you spend time restoring the rest.

This thing will run Yosemite, right?

[Paul F.]

I like deckeda's addition of saving the installer... way better.

[deckeda]

One thing I wasn't clear on, Randalls, the installer saved to a thumb drive is for later, should you need it. As you're already aware, you can just run the downloaded installer as-is from the computer.

Read that MacWorld article. You gotta save it to the thumb drive before you run the installer, or you won't have a downloaded installer to save. Capiche?

[Randalls]

Yes, and according to Everymac this model should support Yosemite.

[deckeda]

So unless you have a specific application you know isn't yet Yosemite-compatible, I'd keep going and get that instead if you can, or after Mavericks is installed.

Yosemite brings any Mac's functional integration a little closer to iDevices running iOS8. If you do that, do the same with Yosemite's installer.

[vision63]

I ran into problems installing on systems with third part ram. http://www.google.com/search?client=safari&rls=en&q=essentials.pkg+yosemite&ie=UTF-8&oe=UTF-8

I had to put the original ram back into my Mini in order to get past that.

[modelamac]

When installing Yosemite or Mavericks, a Recovery Disk is installed. That is the first step when installing Mav/Yosemite, I believe, so it should be there and available on the problem Mac. It's the replacement for the OS on a DVD.

To get to it, hold Cmd R at the startup sound. Then chose to reinstall the OS.

Second question regarding the use of another Mac: The best way to solve that is to start the problem Mac in Target disk mode (hold T at startup sound), and connect it to the good Mac via Firewire. Now the good Mac can install the OS, repair Disk, and do every thing but prepare dinner to the problem Mac.