How do I migrate data from a 3TB iMac to a 256GB Mac mini using an external SSD?

Depending on the apps that you use, a very large amount of space can be taken up in the hidden User Library folder. Some apps will store massive amounts of data there in the form of databases, caches, libraries, etc. Many of these just continue to grow as you use the apps. These unfortunately cannot be located anywhere else other than on the internal drive.

One other thing unrelated to space, coming from a 2012 Mac, you can have a lot of apps that will not be compatible with the latest operating system and with the Apple designed CPU n the new Mini. You need to verify that all your critical apps are compatible and if not upgrade to newer versions.

If that is a new Mac mini that you purchased directly from Apple, and it is less than 14 days since purchase, take it back for a refund and get a Mac mini with at minimum 1 TB and probably a larger SSD depending on how much storage was consumed on the old iMac. You will have to explore return/refund issues with a non-Apple source of purchase.

It depends on how much data you have – of what types – and how much work you want to put into things.

It is theoretically possible to put your Music, TV, and Photos libraries on an external drive. I bought a Refurbished Mac that had only 512 GB of internal SSD space, so didn't even try to move over my iTunes Library stuff until I had bought a nice, large, external SSD on which to store it.

Likewise, if you use Adobe Lightroom Classic, you might find that keeping catalog and cache files on a fast drive – like your internal SSD – is a good idea; but that you can store full-size photo files just about anywhere.

If you can separate out enough bulk data and put it on external drive(s), that might make it so that you could use Migration Assistant to move over the rest to your new startup drive.

That said, even if you're going to be keeping a lot of data on external drives, you might be happier with a Mac mini that has 512 GB or 1 TB of internal SSD space, than one that has only 256 GB. You want the internal drive to have plenty of room to store the OS, applications, and basic user data – with some free space left over.

Hanswizz wrote:

What are everyone’s views on the memory taking into account that I’m coming form a 2012 iMac …Memory: 32 GB …

On Apple Silicon Macs, you cannot expand RAM after purchase. You must get all of the RAM that you will ever need up front. The RAM must serve for the needs of the GPU as well as the needs of the CPU.

You can get

• a Mac mini with a plain M4 chip with 16, 24, or 32 GB of RAM.
• a Mac mini with a M4 Pro chip with 24, 48, or 64 GB of RAM.
• a Mac Studio with a M4 Max chip with 36, 48, 64, or 128 GB of RAM.

If you have, and are using, 32 GB, now, that might suggest going for 32, 36, or 48 GB of RAM. Possibly 24, but only if you were sure that reducing the amount of RAM would not cause problems.

You have not told us what sort of workloads you run now, so it is impossible to really say how much you should get.

+1 on Viking's post, return the MM and buy one with the same amount of storage as your old Mac. Otherwise your migration is simply going to be a nightmare.