How do I transfer data from MacBook Pro to a new Mac mini M4 using a USB-C to Thunderbolt 3 data cable?

That mid-2015 MacBook Pro has old-style Thunderbolt 2 ports with Mini DIsplayPort connectors. Thus in order to make a Thunderbolt connection, you would need a $50 Apple Thunderbolt 3-to-2 adapter and a $30+ Thunderbolt 1/2 cable (a type of cable that is getting difficult to find).

I don't know if Migration Assistant would work with a "plain" USB (not Thunderbolt) connection, but updated rules for Target Disk Mode indicate that it wouldn't. So Migration Assistant might not either.

My suggestion would be to migrate using an up-to-date backup (Time Machine backup or bootable clone backup) of the 2015 Mac on an external drive. Then you're not spending $80+ on Thunderbolt connection hardware which will likely only see one use. Even if you had to buy, say, a new external SSD to make a new backup of the old Mac, you could buy it with the idea of reusing it as a backup drive for the new Mac going forward. In that way, you'd get more use out of your money.

PLEASE just follow the directions in Setup Assistant and Migration Assistant, this the fastest and most reliable method!!!!!!!!!!!!!!

Remember that your new Mac must have at least the same amount or more storage as the old Mac otherwise you cannot migrate. This is critical so please don't ignore it!!!!!!!!!!!!

My basic advice for migration has always been:

1) Do not attempt to migrate the entire computer via WiFi. It is a recipe for disaster. Similarly don’t try via ethernet cable. For whatever reason, migration assistant just doesn’t seem to play nice with network protocols.

2) using target disk mode is safe and reliable and if you are lucky enough to have an old computer, the new computer, and a compatible cable all on hand, that is a good choice. But if you have to invest in a cable which you may only be using once, think twice. And then think again.

3) my choice for migration ever since Apple killed off the FireWire cable, has been to back up from a TimeMachine (or other compatible) backup source. The reason is simple: First, you should already have a current backup on an external hard drive. If not, that’s where you want to spend a little extra money, not on that above mentioned cable. Second, it is reliable

If you don’t already have a backup drive, take a look at how much of your current hard drive is actually being used. For example, 388GB of my 512GB drive is being used. Then increase that number by 50-100% and round up. Due to the way TimeMachine works, that’s how big your TimeMachine drive should be. In my example a 1T backup drive was my choice. Rotational or SSD are both acceptable. I chose an enclosure from OWC and made my own drive.

Your welcome..

Good idea to create a new Time Machine backup on the 5T SSD.

However instead of connecting the 5T SSD to the slower networked Time Capsule.

Connected the 5T SSD directly the old Mac and make a new Time Machine backup.

see > Back up your Mac with Time Machine - Apple Support

Then connect the 5T SSD to the new Mac during the initial setup and Migrate from it.

and > Transfer to a new Mac with Migration Assistant - Apple Support

HootieUK wrote:

to ensure good data transfer rather than over WiFi. I also have a Time Capsule backup of the Macbook but that is similarly aged and could be slow I expect. What am I looking for and source?

Wi-Fi is slow and unreliable.

Using a Time Machine or Clone Backup on an external drive or Time Capsules is the Easiest, Fastest and most Reliable way to move to a new Mac.

Please follow the directions in the link that rfaufmann87 supplied.

Also in terms of connectivity, is it best to data transfer first before attaching the keyboard and peripherals?

You need a Display, Keyboard & Mouse to setup the new Mac mini and then Migrate from your backup.

see > Set up your Mac mini - Apple Support

This is the best solution. Then going forward connect this drive to the new computer whenever you want to do a new backup. I connect my MBAir every Saturday. Time Capsule was very reliable but for whatever reason drives attached to a router just don't work for TimeMachine backups long term.