Sounds like you are not specifying a drive at all. Just specify the drive name after fdisk.
The system startup will show the drives on the system as in the following:
ad0: 955MB <transcend 20071207="">at ata0-master UDMA33
. . .
ad2: 955MB <transcend 20071207="">at ata1-master UDMA33
(I have two 1GB Transcend DOM "hard drives" on my system and they are named /dev/ad0 and /dev/ad2.)
I suggest you get to shell command mode and type the command: dmesg | grep ad which will display the startup output which includes the character string ad
Here's what it produces on my system:
dmesg | grep ad
FreeBSD is a registered trademark of The FreeBSD Foundation.
padlock0: No ACE support.
agp0: Invalid aperture setting 0x0agp0: Invalid aperture setting 0x0agp0: bad aperture size
ath0: Ethernet address: 00:19:e0:68:31:4b
ath0: mac 7.9 phy 4.5 radio 5.6
rl0: Ethernet address: 00:30:18:b0:50:fb
uhub0: <via 1="" 9="" uhci="" root="" hub,="" class="" 0,="" rev="" 1.00="" 1.00,="" addr="">on usb0
uhub1: <via 1="" 9="" uhci="" root="" hub,="" class="" 0,="" rev="" 1.00="" 1.00,="" addr="">on usb1
uhub2: <via 1="" 9="" uhci="" root="" hub,="" class="" 0,="" rev="" 1.00="" 1.00,="" addr="">on usb2
uhub3: <via 1="" 9="" ehci="" root="" hub,="" class="" 0,="" rev="" 2.00="" 1.00,="" addr="">on usb3
rum0: <ralink 0="" 2="" 802.11="" bg="" wlan,="" class="" 0,="" rev="" 2.00="" 0.01,="" addr="">on uhub3
rum0: Ethernet address: 00:1b:11:b5:c1:c8
vr0: Ethernet address: 00:30:18:b0:19:48
udav0: <shantou 0="" 2="" st268="" usb="" nic,="" class="" 0,="" rev="" 1.10="" 1.01,="" addr="">on uhub0
udav0: Ethernet address 00:60:6e:30:32:26
udav0: Ethernet address: 00:60:6e:30:32:26
ad0: DMA limited to UDMA33, device found non-ATA66 cable
ad0: FAILURE - SET_MULTI status=51 <ready,dsc,error>error=4 <aborted>ad0: 955MB <transcend 20071207="">at ata0-master UDMA33
ad2: DMA limited to UDMA33, device found non-ATA66 cable
ad2: FAILURE - SET_MULTI status=51 <ready,dsc,error>error=4 <aborted>ad2: 955MB <transcend 20071207="">at ata1-master UDMA33
Trying to mount root from ufs:/dev/ad0s1a
bridge0: Ethernet address: 92:a5:c3:d6:8d:2f
bridge0: Ethernet address: 36:0f:23:1a:bd:cf</transcend></aborted></ready,dsc,error></transcend></aborted></ready,dsc,error></shantou></ralink></via></via></via></via>
Then you will know the valid parameter values to the fdisk command; on my system fdisk /dev/ad0 or fdisk /dev/ad2</transcend></transcend>