I have a Transcend 256GB SSD370 which i've been trying to run on my iMac I initially installed Yosemite on it and booted from it via a USB HDD enclousre to see if it worked, which it did. I then proceeded to open up the iMac and swap the internal HD with the SSD, when I powered it up it worked. However, after a few minutes it would start umbrella and freeze, after a restart it took ages to load up, when it did it started to freeze and umbrella again. It then refused to boot from the SSD, and would just grey screen and nothing. I've updated the firmware to the latest and even wiped it and tried again, checked the status on my Windows machine and it worked fine. Not sure what else to try?