Today I used BlackMagic for the first time. OS Mavericks. iMac late 2013. i fired it up and tested two ext HDs, both connected via USB 3. It wasn't surprising that my 2Tb 3.5 inch Toshiba clocked an I/O speed roughly 2X that of a smaller 500Gb Toshiba. The 2Tb is a boot drive and has BlackMagic in the Application folder. Why won't it allow me to test the main drive on my computer. (while booted on the ext TB)? And to go further, suppose I bought an SSD and connected via USB 3. Would the I/O speeds clock faster than any spinning disk? Thanks. This stuff is exciting, but beyond my understanding.