Hello mollie164,
Thanks for using Apple Support Communities. I saw your post about Migration Assistant, and I'm happy to help out.
It sounds like there may be a few factors to consider here. First, which macOS versions are you running on both Mac? You can check for the specific versions under Apple menu () > About This Mac.
Have you tried to run First Aid on the external drive you use for Time Machine? It sounds like that may be your best starting point to see if this resolves the backup issue: Repair a disk using Disk Utility on Mac. I also recommend using the same process on your internal hard drive to ensure there are no errors there. If you're able to complete the backup, you should then be able to migrate accordingly.
Regarding the Migration Assistant methods you tried, can you provide more information about the type of cable connection you used? You mentioned an HDMI cable, but you may want to double-check to see if you have one of the below available instead. This is from the "Connect your computers with the correct cable" section of the resource titled Use target disk mode to move files to another computer:
"Identify the ports on both Mac computers so that you can choose the correct cable. Target disk mode works when your computers are connected to each other using any of these ports:
- Thunderbolt 3 (USB-C)
- USB-C
- Thunderbolt 2
- FireWire
- Target disk mode doesn't support connections that use an Apple USB-C Charge Cable, USB-A to USB-A cable, or Mini DisplayPort
cable.
If one or both computers has a Thunderbolt 3 (USB-C) or USB-C port, you can connect them as follows:
I hope this helps you!
Cheers.
