fdisk fails with "Class not found"

[Bob Willcox]

I am trying to change the sysid of a slice (partition) on my disk from 5 to
165.  When it gets to the point of writing sector 0 it always fails with:

fdisk: Class not found
fdisk: Failed to write sector zero

Can someone please tell me what this means, and how do I fix it?
I have included the output from an fdisk attempt below.

Thanks,
Bob

root at tavion:10 /usr/home/bob> fdisk -uv
******* Working on device /dev/ad4 *******
parameters extracted from in-core disklabel are:
cylinders=969021 heads=16 sectors/track=63 (1008 blks/cyl)

Figures below won't work with BIOS for partitions not in cyl 1
parameters to be used for BIOS calculations are:
cylinders=969021 heads=16 sectors/track=63 (1008 blks/cyl)

Do you want to change our idea of what BIOS thinks ? [n] 
Media sector size is 512
Warning: BIOS sector numbering starts with sector 1
Information from DOS bootblock is:
The data for partition 1 is:
sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
    start 63, size 524286945 (255999 Meg), flag 80 (active)
        beg: cyl 0/ head 1/ sector 1;
        end: cyl 1023/ head 15/ sector 63
Do you want to change it? [n] 
The data for partition 2 is:
sysid 5 (0x05),(Extended DOS)
    start 524297340, size 452470725 (220932 Meg), flag 0
        beg: cyl 1023/ head 255/ sector 63;
        end: cyl 1023/ head 14/ sector 63
Do you want to change it? [n] y
Supply a decimal value for "sysid (165=FreeBSD)" [5] 165
Supply a decimal value for "start" [524297340] 
Supply a decimal value for "size" [452470725] 
Explicitly specify beg/end address ? [n] 
sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
    start 524297340, size 452470725 (220932 Meg), flag 0
        beg: cyl 968/ head 4/ sector 1;
        end: cyl 311/ head 14/ sector 63
Are we happy with this entry? [n] y
The data for partition 3 is:
<UNUSED>
Do you want to change it? [n] 
The data for partition 4 is:
<UNUSED>
Do you want to change it? [n] 
Partition 1 is marked active
Do you want to change the active partition? [n]  

We haven't changed the partition table yet.  This is your last chance.
parameters extracted from in-core disklabel are:
cylinders=969021 heads=16 sectors/track=63 (1008 blks/cyl)

Figures below won't work with BIOS for partitions not in cyl 1
parameters to be used for BIOS calculations are:
cylinders=969021 heads=16 sectors/track=63 (1008 blks/cyl)

Information from DOS bootblock is:
1: sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
    start 63, size 524286945 (255999 Meg), flag 80 (active)
        beg: cyl 0/ head 1/ sector 1;
        end: cyl 1023/ head 15/ sector 63
2: sysid 165 (0xa5),(FreeBSD/NetBSD/386BSD)
    start 524297340, size 452470725 (220932 Meg), flag 0
        beg: cyl 968/ head 4/ sector 1;
        end: cyl 311/ head 14/ sector 63
3: <UNUSED>
4: <UNUSED>
Should we write new partition table? [n] y
fdisk: Class not found
fdisk: Failed to write sector zero

-- 
Bob Willcox                  Stock's Observation:
bob at immure.com                   You no sooner get your head above water
Austin, TX                       but what someone pulls your flippers off.