I tried the script, provided by RubinMac - it did the job however it is not as accurate as I thought it to be.
I enhanced the script and had 3 variance thresholds
100 > Variance > 150
150 > Variance > 200
All others were moved to Ok.
I found few of the pics placed in the above three criteria were actually very clear and couldn't find the ...
You say that you can't connect all of the drives at once, but do you have enough ports to connect two at a time? If you do, what about having one drive dedicated to storing the catalog and the previews, which you always connect to the computer you're using? Then you can also connect the drive containing the specific photos when needed.
If you're short of USB ...
As I see it you have two problems:
Have the edits with the photos on the SSD
Have the previews on all the different machines
To solve problem 1 you can write edits to XMP files, as described in this answer. Then the edits will be stored next to your photo files and readable by every machine connecting to the SSD.
To solve problem 2 you can have a catalog ...
The standard Undo and Redo keyboard shortcuts (Ctrl-Z and Ctrl-Y on Windows and Linux, CMD-Z and CMD-shift-Z on Apple) operate in Darktable - at least, they do running in Ubuntu : I haven't checked the other OSs. More Darktable shortcuts here.
If you're looking for a way to view the image with and without a specific module effect, each module has a "power ...