If I enabled vmm in /boot/loader.conf and added some entries like pptdev="1/0/0", hot swap became unusable. The corresponding entries in /var/log/messages are Aug 24 19:50:54 aaa kernel: ahcich1: Poll timeout on slot 2 port 15 Aug 24 19:50:54 aaa kernel: ahcich1: is 00000000 cs 00000000 ss 00000000 rs 00000004 tfd 150 serr 00000000 cmd 00710217 Aug 24 19:50:54 aaa kernel: (aprobe0:ahcich1:0:15:0): NOP FLUSHQUEUE. ACB: 00 00 00 00 00 00 00 00 00 00 00 00 Aug 24 19:50:54 aaa kernel: (aprobe0:ahcich1:0:15:0): CAM status: Command timeout Aug 24 19:50:54 aaa kernel: (aprobe0:ahcich1:0:15:0): Error 5, Retries exhausted Aug 24 19:51:09 aaa kernel: ahcich1: Poll timeout on slot 4 port 0 Aug 24 19:51:09 aaa kernel: ahcich1: is 00000000 cs 00000000 ss 00000000 rs 00000010 tfd 150 serr 00000000 cmd 00710417 Aug 24 19:51:09 aaa kernel: (aprobe0:ahcich1:0:0:0): NOP FLUSHQUEUE. ACB: 00 00 00 00 00 00 00 00 00 00 00 00 Aug 24 19:51:09 aaa kernel: (aprobe0:ahcich1:0:0:0): CAM status: Command timeout Aug 24 19:51:09 aaa kernel: (aprobe0:ahcich1:0:0:0): Error 5, Retries exhausted With the line on pptdev commented out and then reboot the system, hot swap was again usable and in /var/log/messages Aug 24 18:53:14 aaa kernel: ada6 at ahcich1 bus 0 scbus1 target 0 lun 0 Aug 24 18:53:14 aaa kernel: ada6: <VK0800GDJYA 4IWVHPG0> ATA8-ACS SATA 3.x device Aug 24 18:53:14 aaa kernel: ada6: Serial Number XXXXXXXX Aug 24 18:53:14 aaa kernel: ada6: 600.000MB/s transfers (SATA 3.x, UDMA5, PIO 512bytes) Aug 24 18:53:14 aaa kernel: ada6: Command Queueing enabled Aug 24 18:53:14 aaa kernel: ada6: 763097MB (1562824368 512 byte sectors) Aug 24 18:53:33 aaa kernel: ada6 at ahcich1 bus 0 scbus1 target 0 lun 0 Aug 24 18:53:33 aaa kernel: ada6: <VK0800GDJYA 4IWVHPG0> s/n XXXXXXX detached Aug 24 18:53:33 aaa kernel: (ada6:ahcich1:0:0:0): Periph destroyed The above behaviour is the same whether I pass through a vga card or a usb 3.0 device. The motherboard in question is ASRock C226WS and the SATA controller is a Marvell 9172.