On AWS, when hot adding an EBS volume to an EC2 instance, FreeBSD doesn't auto-detect the hot-add event.
Manually running "devctl rescan pci0" will detect the newly added device.
Rebooting the instance (which also forces a pci rescan at boot) will also work.
12.1-RELEASE-p6 GENERIC amd64
FreeBSD 12.1-RELEASE-amd64-f5af2713-6d84-41f0-95ea-2eaee78f2af4-ami-0de268ac2498ba33d.4 - ami-05c3afd6554d42483
Root Device Type: ebs Virtualization type: hvm