As will have debian unstable. That’s the way it goes, for a few months every few years it slows down until the new stable gets released. Testing is just 10 days after unstable to avoid the biggest bugs.
Never had big problems with debian unstable in 15 years though, as long as you use apt-listbugs
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.