Problem seems to appear when CF disk is formatted in windows operating system, to fix this you can use diskpart util.

To do that start "Command prompt" as "Administrator" and do following. Note in my case CF card was disk 3.

WARNING MAKE SURE YOU SELECT RIGHT DISK AS EVERYTHING IS GOING TO BE WIPED OFF..

Microsoft Windows [Version 6.1.7601]
Copyright 2009 Microsoft Corporation.  All rights reserved.

C:\Windows\system32>diskpart

Microsoft DiskPart version 6.1.7601
Copyright (C) 1999-2008 Microsoft Corporation.
On computer: PC

DISKPART> list disk

Disk ###  Status         Size     Free     Dyn  Gpt
 –------  -------------  -------  -------  ---  ---
 Disk 0    Online          119 GB      0 B
 Disk 1    Online          931 GB      0 B        *
 Disk 2    Online          931 GB      0 B
 Disk 3    Online         3815 MB      0 B

DISKPART> select disk 3

Disk 3 is now the selected disk.

DISKPART> clean

DiskPart succeeded in cleaning the disk.

DISKPART>

and now you should be able to write to the CF card

c:\write>physdiskwrite -u pfSense-2.0.2-RELEASE-2g-i386-nanobsd.img.gz

physdiskwrite v0.5.2 by Manuel Kasper mk@neon1.netSearching for physical drives…

Information for \.\PhysicalDrive0:
  Windows:      cyl: 58756
                  tpc: 224
                  spt: 19
  C/H/S:        16383/16/63
  Model:        SAMSUNG SSD 830 Series
  Serial number: S0Z3NEAC845095
  Firmware rev.: CXM03B1Q

Information for \.\PhysicalDrive1:
  Windows:      cyl: 121601
                  tpc: 255
                  spt: 63
  C/H/S:        16383/16/63
  Model:        ST31000528AS
  Serial number:            9VP55BRK
  Firmware rev.: CC38

Information for \.\PhysicalDrive2:
  Windows:      cyl: 121601
                  tpc: 255
                  spt: 63
  C/H/S:        16383/16/63
  Model:        WDC WD10EARS-00Y5B1
  Serial number:      WD-WCAV5H808061
  Firmware rev.: 80.00A80

Information for \.\PhysicalDrive3:
  Windows:      cyl: 486
                  tpc: 255
                  spt: 63

Which disk do you want to write? (0..3) 3
WARNING: that disk is larger than 2 GB! Make sure you're not accidentally
overwriting your primary hard disk! Proceeding on your own risk...
About to overwrite the contents of disk 3 with new data. Proceed? (y/n) y
Found compressed image file
1989969408/1989969408 bytes written in total

c:\write>

Cheers
Jouko/mk@neon1.net