Suite à la discution de ce midi :
$ uname -a
OpenBSD xxx 4.6 GENERIC#1 i386
$ sudo mount /dev/sd0i /mnt
$ dmesg | tail
ums0 at uhidev0: 3 buttons, Z dir
wsmouse1 at ums0 mux 0
umass0 at uhub1 port 1 configuration 1 interface 0 "ChipsBnk USB Reader"
rev 2.00/1.00 addr 3
umass0: using SCSI over Bulk-Only
scsibus0 at umass0: 2 targets, initiator 0
sd0 at scsibus0 targ 1 lun 0: <ChipsBnk, SD/MMCReader, 4081> SCSI2
0/direct removable
sd0: 3774MB, 512 bytes/sec, 7729152 sec total
sd0 detached <--- ici on arrache la clef
scsibus0 detached <--- incroyable, ça ne panique pas
umass0 detached <--- :p