OK. failure!
1. Deleted Guest account in Fusion. Opened up 2007 iMac and connected via USB3.0 cable, (I have no USB2.0). I created a new Lion Guest iso and installed it. Set up process to Migration Assistant; ensured 2007 iMac Migration Assistant was set ‘To another Mac’ then tried to select source which Guest Lion could not find.
2. Noting that Migration Assistant asked for the computer to be connected to same network via Ethernet I ditched USB and connected via Ethernet. No source computer found! I checked Ethernet connection and found the source computer confirmed connection as did the 2019 iMac.
3. Created a new Guest Account running Yosamite 10.10.2. Tried Migration Assistant as above with both USB3.0 and Ethernet cables. Source disks could not be found.
I do not have a USB2.0 dock but with USB2.0 ports on the 2007 iMac directly connected that should be irrelevent? The USB 3.1 cable should work as it is backwards compatible. My old external LaCie drive connectivity is either Firewire 400 and 800, or USB3.0.
Clue: Although Ethernet showing connected on 2007 iMac, Connection is automatic on Ventura 2019 iMac. Settings on 2019 Ventura merely show Self Assigned IP with Yellow not Green highlight.
In Ventura I cannot find how to manually assign the 2007iMac IP Address. Given Migration Assistant requires Ethernet this is the current problem.
