Hi, I am trying to use iMacs available in my school's library as an external display for my macbook pro. The first time it worked, but I have no success since then. I am wondering if you can help me troubleshoot. My macbook pro is from mid 2013, and the iMac is from mid 2011. Both have thunderbolt ports. I bought thunderbolt cables to connect them. The first time I tried, it worked right away. I didn't even had to press CMD + F2 as suggested in the apple website. But since then it is not working anymore. When I connect the thunderbolt cable in either of the ports, the system report on the thunderbolt port of my macbook reads "no device connected" I have tried the following to troubleshoot this problem: - Tried with many different iMacs in the same library - Tried different cables with those iMacs - Tested and ensured that one of the cables were defective but I have at least one working cable. I ensured it by trying the cable with a 27 thunderbolt display - I have reset the SMC and NVRAM of my macbook pro - I contacted the it support of the library but they mentioned that the iMacs have no such things as thunderbolt port. Which is clearly wrong because it worked through the thunderbolt cable the first time I am wondering if there is anything else I can try to solve this problem decisively. How can I determine the origin of the problem? If it is being blocked by the system admin after my first try, is it possible to check it by reading the system report? I am assuming that I do not need to reset the SMC and the NVRAM of the iMacs because it is not working in either of them and it seems unlikely that all of them would require those resets. What to try next?