imac freezing

And Safe Boot doesn't help?

1. Start up from macOS Recovery

To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following combinations on your keyboard. Command-R is generally recommended, especially if you never installed macOS Sierra 10.12.4 or later… https://support.apple.com/en-us/HT204904#notupdated

Command (⌘)-R

Install the latest macOS that was installed on your Mac.

Option-⌘-R

Upgrade to the latest macOS compatible with your Mac.

Shift-Option-⌘-R

Install the macOS that came with your Mac, or the closest version still available.

I have a similar problem. Brand new imac, started up, updated, then performing migration assistant from my late 2009 imac (which runs on High Sierra) when it froze stating 4 minutes to go for an hour or more.

Cancelled migration assistant and started it again but this time it froze on "collecting file information".

Cancelled migration assistant again, tried to reboot, but cant get past the black startup screen with the white apple and the progress bar. Looks like gets to the end of the progress bar and then just stops altogether. Left overnight with no change at all.

Thanks. While I was waiting for this reply I engaged an Apple Support Chat who took me through the same process, though ultimately I had to do an "on-line reboot" (or something like that), erase my disc using Disc Utilities, and then reinstall . Seems to have fixed the problem and now working through the migration assistant again, this time from my Time Machine portable disc drive rather than direct from my old Mac.

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

Any luck yet?

Good to hear, take care my friend. :)