Replaced OWC Mercury Accelsior E2 that I am using as a start up disk and it keeps disappearing
When updating to Yosemite you will want to make sure that you are at least running 10.6.8. Running the early versions of 10.6 when updating to Yosemite, will cause errors.

You will also want to make sure that the TRIM has been disabled. Please take a look at this OWC Blog post for instructions: http://blog.macsales.com/27116-disable-trim-before-upgrading-to-yosemite

I am a member of management with Other World Computing