Does it only happen with these two drives? i would try with some other HDD/SSDs or two usb sticks. that way you can test if its some weird hardware incompatibility that sometimes happen between specific devices or if the board wont support more then one connected usb drive in general.
In the past there were mutliple tools: apt-cache (searching packages), apt-get (managing packages), apt-file (searching for files belonging to packages), apt-key (managing repository keys)…
A few years ago some developers created apt to combine these multiple tools into the single program called apt. Both tools (the old apt-… and the new apt) use dpkg in the backend to install and remove packages. Looks like apt hasn’t done its documentation homework.