Method 1. First, use the time machine to back up the mechanical hard disk to the mobile hard disk, then replace it with ssd, and press command+r to start the computer. Download the osx tool from the network first, and then use the "Backup from Time Machine" in the tool to restore it. It seems that this method is widely used, and I intend to use it at first, but the problem with this method is that downloading osx tools depends on the network, and it may take a long time if the network speed is slow or the Apple server is busy. But it should be possible to switch to a USB flash drive to start the solution.
Method two. Load the ssd that needs to be replaced into the hard disk box, connect the mini, and copy the disk with "Restore" in Disk Utility. It is said that this method is error-prone and not very good. And ssd didn't restore hd partition. In addition, the capacity of ssd must be equal to the mac partition of mechanical hard disk, which is why I didn't try this method.
Method three. After replacing the ssd, first install a new system on the ssd with a USB flash drive or other methods, then put the old hard disk into the hard disk box, connect the mini, and use the tool of "migration assistant" to overwrite the old system on the old hard disk with the new system on the ssd.
Method four. As in Method 2, load the ssd to be replaced into the hard disk box and connect the mini. Then use the tool to copy the clone directly, without the disadvantages of method 2. This should be the most convenient method and the last method I adopted. At present, ssd, as an external hard disk, has been successfully started and should be disassembled and used.