Restore a data partition on a new mac through migration assistant

I think the problem is that Migration Assistant looks only for drives which have an operating system on them and thereby, also a Migration Assistant app to find and connect to. Your other partitions do not have operating systems installed in them. It might be a bit slow and laborious but why not simply copy the files over to the new Mac from the two partitions?

Yowannes wrote:

I appreciate your feedback though: I looked up some information about the APFS volumes. Although I don't really see much advantages (except maybe for the flexibility on disk space, although not really an issue to me), I also don't see any disadvantages either. So I could perfectly switch to the APFS way of working.

What exactly is risky about using partitions instead of the APFS volumes, please? As I couldn't find anything on risk of partitions.

Partitions are risky when you create new ones since the existing partition and its volume must be resized. Same thing goes for resizing partitions later on when you realize one of them is too small. If you have three or more partitions, then it may be impossible to resize the partitions because they are not next too each other....in that case you would need to delete those other partitions & recreate them with different sizes & restoring their data.

You may not think space is an issue at this time, but a few years down the road you may discover it is a big issue. By then you are already fully committed to the partitions so making any changes is very risky and perhaps even a time consuming project. Even if APFS volumes were not an option, I would still recommend avoiding multiple partitions on any drive boot or data. Much better to dedicate another whole drive.

Also keep in mind that if space does become an issue, that deleting an APFS volume is much faster & easier with little risk compared to deleting an entire partition & resizing the other partition & volume.

@MartinR also provided good information as well.

Now, let's say that erase my harddrive again, add the partitions back together and create volumes, how could I then restore my TimeMachine backups into the appropriate APFS volume, please? As I don't see how the volumes would give me a different outcome to my initial question.

macOS doesn't really deal with partitions when it comes to any of the normal software. Only Disk Utility cares about partitions. Everywhere else in macOS....everything is a volume. An APFS volume within a Container is no different to a volume of a partition. Once the volume is mounted, both types are identical to all your normal apps. I don't see that you would have any issues.

I am still in a phase where I can switch to the APFS-volume system (erase drive, remove partitions...), but how would that way of working change my initial question? Is there a way to recover TimeMachine backups to an APFS-volume (which I don't see when trying to restore to a partition like is currently the case), please?

I don't use TM so maybe another more knowledgeable contributor can confirm. I think you need to use the TM app to restore your items to those particular volumes. You cannot use Migration/Setup Assistant to restore the backups to any other volume/partition other than the main boot drive where the home user folder is stored in a default configuration. I have seen some people report issues even migrating from TM backup when they have moved their home user folder.

I hope you did not actually create multiple partitions on your new Mac as that is very risky. With the new APFS file system, you can achieve the same result by creating new APFS volumes. These new APFS volumes will share the same storage pool as the main APFS volumes used by macOS, but don't require you to restrict them in size. Each APFS volume can be mounted or unmounted just like a partition so the data on them is still separated like a partition.

People should never use multiple partitions on a drive because most people discover at some point that one or more partitions does not have enough room.

Add, delete, or erase APFS volumes in Disk Utility on Mac - Apple Support

If this is a new Apple silicon Mac Mini with an SSD you should not partition the boot drive. You will set yourself up for problems sooner or later. You should create additional Volumes on the SSD (inside the APFS container, usually called Container Disk 1).

Things are very different in Monterey & later versions of macOS and on Apple silicon. It's not just a matter of partitions being fixed size vs. volumes being variable size ... it's an entirely new architectural world and these new machines expect a boot SSD to be a single APFS container.

I get it. I understand that it is possible to get a recovery from Time Machine. That said, I prefer not to use Time Machine as it is not very user friendly. (I use carbon Copy Cloner which gives me backups that look exactly like my HD. Time Machine does not.) Very likely someone else here will be able to help you with this. Good luck.