Cables to migrate from 2009 iMac to 2018 Mini

Do not migrate using the method you are attempting! The most reliable method is to connect the old Mac's Time Machine backup (or bootable clone backup) to the new Mac via a USB cable and then launch Migration Assistant (Applications - Utilities - Migration Assistant) on the new Mac. It works every time, assuming the backup is in good shape. If a backup of the old computer has not been done, then create a bootable clone using the free version of SuperDuper!, this will only require an external HD.

Let me second rkaufmann87's advice. Migration via WiFi is useful only for a few GB of data at best. Assuming Migration Assistant didn't stall (most likely it will) a WiFi migration would be an overnight affair at best. Theoretically, migration via Ethernet cable would be the fastest method but stalls are quite common. When Apple released its first computer without FireWire I tested various migration methods and found that migration via Ethernet failed frequently. I came to the conclusion that the only reliable migration method involved an external hard drive. Considering that everyone needs to have a backup plan involving an external drive, this isn't such a bad thing.

If you aren't already using TimeMachine to back up your computer, look at how much drive space your old computer is using now. Multiply that by 2 or 2.5 then round up to the nearest hard drive size. For example, if your drive has 400GB, double is 800GB so a 1T drive is the smallest external drive you'd want and 2T would probably be better. (The bigger the drive the longer TM can make incremental backups before it has to delete something potentially important.)

I posted a quick response waiting for a show to start but thought more about your problem on the way home. $80 for dongles that you'll probably use once isn't a good solution. But I have another possible solution that may cost you nothing or about the same $80 leaving you with an extra external hard drive which isn't useless. Two backups are better than one.

When I purchased my 2011 MBAir I had to figure out how to move about 600GB onto a 256GB SSD. I wound up doing it by cloning my MacBook Pro and then removing things I didn't absolutely need from the cloned drive. Videos, lots of music, old data I hadn't touched for ages, programs I almost never used - deleted them. When I was done the cloned external would fit on the MBAir and I used that external drive as my source for migrating apps and data to the new computer.

SuperDuper! is a shareware cloning program that is free to use for simple clones. I recommend it highly. You can get a decent 2Tb external drive for under $100 and once you've completed its migration tasks it will be valuable as another backup source or for storing files you couldn't fit on the new computer

MA will allow you to select documents, settings & apps, however you cannot select which settings, which apps or which docs. I'm not sure why you go with a smaller drive but that is what you have. So your best bet is to use Target Disk Mode to connect the two computer and manually select the documents & apps you want to migrate.

You may have compatibility issues on some apps particularly if you are use legacy (meaning OLD) versions that have not been kept up-to-date. Before moving your apps, inventory what you have on the old Mac, determine the apps you want to copy over and then ensure these are up-to-date. If they are Apple apps or apps available on the Mac App Store this is easy, however if you have 3rd party apps that can only be updated from the developers website then it can be more tedious and possibly expensive.

Some apps such as MS Office will not allow you to copy them, they must be manually installed and you will need the original security code to install it. They may also NOT install at all if they are really old as the developers won't support them on later versions of Mac OS. For example, if you have MS Office for Mac V2004, it will not install at all. This means if you want to continue with MS Office you have to buy a new version (hey it's 15 years old you got your money's worth) or you can explore alternative apps.

How to move your content to a new Mac - Apple Support

FWIW, since you have been using that old iMac for 10 years, it

may be best to just start fresh on the new iMac doing fresh installs

of your apps and just migrate your user data. BTW, before starting,

make sure any of those apps apps are compatible with macOS Mojave (10.14.x).

Doing a full migration may pull some old things into the new iMac

that may not be compatible and cause issues.

You could migrate applications and settings and one small folder from your account to establish that account. Then you could use time machine after you’ve logged into the computer you could use time machine to pick and choose files to restore.

When you restore using Migration Assistant it you select apps, settings, documents etc. However you cannot discriminate which apps, which docs etc. that is all or nothing.