completely OT

[James Linder]

Hi

I have a mid 2011 iMac. I setup a SSD on usb3 over thunderbolt. 
All good EXCEPT that it wont boot. Will boot on the usb2 ports but they are slower than usb3. 
I'm looking for a pointer or advice on how to boot on diskA say the internal disk, but run (root on) diskB 
I can't find a thunderbolt 1 disk or adapter 

Can anybody point me at where I could ask these sort of questions
sorry for last-resort posting here

Thanks
James