I have a iMac mid 2010 with a mini display port that I would like to use as a second screen for my new iMac with a thunderbolt 3 port.
First question is can I do that for a display?
Second questions is can I access the data on my old Mac?
iMac 21.5″, macOS 10.13
1) Yes, if the iMac is 27". You will need a USB-C to Mini DisplayPort cable, such as:
mophie USB-C Cable with Mini DisplayPort Connector - Apple
Use your iMac as a display with target display mode - Apple Support
( The "Mac from 2019 or earlier" is for Thunderbolt connection, and doesn't apply for your Mini DisplayPort 2010 iMac.)
2) You can use Target Disk Mode.
Transfer files between two Mac computers using target disk mode - Apple Support
1) Yes, if the iMac is 27". You will need a USB-C to Mini DisplayPort cable, such as:
mophie USB-C Cable with Mini DisplayPort Connector - Apple
Use your iMac as a display with target display mode - Apple Support
( The "Mac from 2019 or earlier" is for Thunderbolt connection, and doesn't apply for your Mini DisplayPort 2010 iMac.)
2) You can use Target Disk Mode.
Transfer files between two Mac computers using target disk mode - Apple Support
The point of Target Display Mode is that your 2010 iMac would display the screen of your new iMac using the keyboard and mouse of the 2010 iMac to control the new iMac. That is not what I think you are asking. I believe you want to treat that 2010 iMac as a dumb secondary display — in addition to your new iMac, and that dog won't hunt, because it is a computer, not an ordinary monitor.
If you want to expand your Desktop on the new iMac, then purchase a quality secondary display and connect to it with a DisplayPort cable (for maximum resolution). When you do this, and power on that secondary display, you will see an Arrangement offering in the Displays panel. From that, you can configure the iMac to have an extended desktop onto the external display.
second display
