@masgo I think you can do these things:
Use something like DropBox and have your knime-workspace there. If you have finished your work on one PC make sure everything is saved, KNIME has been closed and the sync is complete. Then you can use the 2nd PC, wait till DropBox has done the sync and the open KNIME. This does work and has the additional benefit of creating a backup and a version control.
If you store a workflow with its data (green traffic light) everything should be fine as log as you give the sync enough time. Also you have to make sure that your Cloud provider would accept some special characters like # in the file names (there have been issues with some settings of OneDrive for companies in the past).
The next thing you could do is indeed use and external drive. Here it is key that the connection ist stable and you shut down KNIME gently and unplug the drive in a gentle way. The principle would be the same.
I would recommend you try the DropBox solution. I have used this with MacOS and windows for some time. Just make sure only one instance of KNIME is active at the same time.