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Helpful answers
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Aug 11, 2016 4:03 AM in response to jlauerscby lllaass,★HelpfulDo both Macs have a Thunderbolt port? The older Mac may have a mini displayport which is used for video only but accepts a Thunderbolt plug since mini displayport video is a subset of Thunderbolt.
Also, are you using a Thunderbolt cable and not a mini displayport cable? They look the same at first glance but the Thunderbolt plugs should have a lightning icon on each plug.
Also, if you look in System profiler
OS X: About System Information and System Profiler - Apple Support
under Thunderbolt what does that show as being connected?
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Aug 11, 2016 4:03 AM in response to lllaassby jlauersc,Thanks for feedback - yes, both MacBooks have 2 thunderbolt ports, cable is correct (even replaced it thinking it broken). Have a thunderbolt adaptor for exterior monitor that works on both computers. when i connect both laptops with thunderbolt cable, system profiler indicates no device connected at thunderbolt port.
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Aug 11, 2016 6:31 AM in response to jlauerscby lllaass,On both Macs try:
- Try resetting the SMC and NVRAM/PRAM
Intel-based Macs: Resetting the System Management Controller (SMC)
https://support.apple.com/en-us/HT201295
About NVRAM and PRAM
https://support.apple.com/en-us/HT204063
- Try starting in Safe Mode
OS X: What is Safe Boot, Safe Mode?
https://support.apple.com/en-us/HT201262
- Boot to Recovery and repair the startup disk
OS X: About OS X Recovery - Apple Support