Thank you for your answers!
This is what I was told by Dxo:
"Thank you for writing and for the question. Whether you can use photo files stored on daisy-chained hard drives depends on the drives and type of connection you use. DxO PhotoLab is a very high performance program. It will make demands on your computer that other post production programs will not make because it is proactive in nature, not reactive. That is, as soon as you select a folder to use, the program begins importing the files in the folder and automatically starts processing the files. This processing consists of analyzing the information in each individual file, creating a unique set of processing settings for each file, and rendering the files for use in the program. Because of this, there must constantly be a very high level of data transfer between the files and the program. In many cases, daisy-chaining files together can result in good data communications for the first drive, but increasing slower transfer speeds for the drives further down the chain. Normally, we recommend that only drives that are directly connected to the computer be used with PhotoLab in order to maintain the highest level of communication between the drives and the program. The use of daisy-chained drives can result in the program not being able to process your photos.
Get a drive with the highest transfer rate and reliability you can find. As we noted, DxO PhotoLab 2 is a very high performance program. If data is not provided to it in a timely manner from the system, this can cause processing and compatibility errors. We do not have a minimum transfer speed to recommend because, again, everyone’s needs and systems are different.
Also, get a system with as much system memory as you can afford, and as good a video card with as much video memory as you can afford. Remember, you are not just purchasing a system to run PhotoLab 2, no one does that. You are purchasing a system to run all of the programs you normally use."
This answer from Dxo makes me wonder what kind of external drive is best to use. I have to buy a new iMac soon. The internal ssd disks have little storage capacity, so I have to buy external drives too. What is the best solution? Just ssd-disks? I have almost 2 tb of photos… Perhaps a fast working drive (ssd) and an ordinary hard-drive ( large storage capacity backup-drive)?