how to transfer data from old iMac to a mac mini m1?

Hello,

Which OS version are you running on the old iMac? New, I think it would have been in the Yosemite or El Capitan range so both the iMac and Mac mini would need to be one the same Wi-Fi network and near each other to transfer wirelessly. Below I coped the pre-transfer information from the article you asked this question from. If you have tried it unsuccessfully I think steps 3 or 4 could be to blame.

Move your content to a new Mac - Apple Support

"Get ready

1. Install all available software updates on both computers. If your old Mac isn't using OS X Lion or later, and your new Mac isn't using OS X Mavericks or later, follow the Mountain Lion migration steps instead.
2. If both computers are using macOS Sierra or later, place them near each other with Wi-Fi  turned on. If either one is using OS X El Capitan or earlier, make sure that both are on the same network.
3. On your old Mac, choose Apple menu  > System Preferences, then click Sharing. Make sure that a name appears in the Computer Name field.
4. If you installed antivirus or firewall software on either Mac, turn that software off until after migration is complete."

I hope this helps.

Simple, backup the 2008 using Time Machine. Then connect the Time Machine External Hard Disk to the Mac Mini and run Migration Assistant.this is the easiest, most reliable and overall quickest method. Try another method and get ready for trouble!

The old Mac cannot be used as an external display, please do not waste time trying to make it work. MM’s are great but they do require an external display, not another computer!

Please remember all Macs are computers, they are not external displays for other devices including other Macs.