Hi - I guess there are better message boards for this, but - under the helpful advice of some nice people here, I just purchased a Crucial BX100 SSD for my perfectly-healthy 2008 aluminum Macbook. I am not a techie, but it didn't seem too difficult. I made a bootable USB of Yosemite using diskmakerX. I have my time machine (and crashplan) backups. I didn't have a SATA-USB cord so I didn't make a clone, but I feel adequately backed up with multiple copies of my stuff. Install seemed painless. Everything seemed fine. But when I turn on the computer and hold down the <option> key, it starts up with no notice of my (Yosemite boot) USB. I get the little black arrow icon on a clean white screen. I tried holding both the <option> and the <command> keys: I get the black apple logo (yay!) and progress bar for about 5 seconds - then the computer shuts down. I plugged in the external with the time machine backup, holding down the <option> key. My drive's name and icon pop up (yay!) with an arrow below it. I click on that. I get the black apple logo (yay!) and progress bar for about 5 seconds - then the computer shuts down. I feel like nothing is seriously wrong, but I seriously don't know what to try next. Any ideas? Thanks in advance.