Hi,
rustybird and I have had this discussion on the forum:
https://forum.qubes-os.org/t/udev-rules-not-working-as-before/35258/
TLDR:
In 4.2.4 I was able to attach individual partition of a USB drive and successfully fstrim it in the target qube.
In 4.3.0 that is no longer possible, using the same udev rule. The only way to fstrim it now is to attach the whole USB device to the qube.
Has there been any change in 4.3.0 in regards to that causing that discrepancy?