OSX Yosemite bootable usb drive circle slash ("no" symbol) I have a Late-2011 Macbook Pro (4GB RAM) that has been running slow for several OSX cycles, and since Mavericks it's been unbearable (spinning beachballs whenever you do anything). I tried upgrading to Yosemite hoping this might improve performance; no dice. I've decided to try clean-booting Yosemite to see if this improves things.

I've been following instructions here to do a clean install and here to create a bootable flash drive. Drive creation seems to work fine: it has the correct GUID partition table, terminal command exits with the copy complete; done output, the disk shows up in the Finder as "Install OSX Yosemite", etc. When I restart the computer holding Option, the installer shows up as a bootable disk (unlike the question here).

My problem is that when I then click on the installer, I get a loading bar for some seconds and then the screen goes to a circle with a slash through it. It's the same behaviour detailed here but note that I have verified that my disk has the correct GUID partition table. I also tried resetting PRAM, to no effect.

Any suggestions welcome.
The boot problem seemed to have been related to a disk error on the hard disk of the mac. I ran the disk utility and it detected a problem with the macbook's HDD (something with doubled file links), which it then repaired. The computer was still slow and unresponsive after the disk repair. After that the boot from the USB worked fine, and I was able to clean install OSX Yosemite.

I recovered the computer from my time machine backup, excluding applications, and now the computer is noticeably faster and more responsive. I don't know whether a specific application or accumulated OSX sludge was responsible for the slowdown, but I'm happy I went through the pain of the clean install.