Hi,
Earlier, there was a thread (now expired) in this newsgroup about
identifying disk formats.
I suggested disktype[1] as a way to identify unknown disks, partitions
etc. Recently I had the opportunity to use it myself, while looking
through some old zip disks (yes, you know the old 100MB ones, from the
original zip drive[2]).
Here follows a few examples of what disktype can do for you.

First, I'll try fdisk:
root@music1# fdisk /dev/da0
******* Working on device /dev/da0 *******
parameters extracted from in-core disklabel are:
cylinders=96 heads=64 sectors/track=32 (2048 blks/cyl)

parameters to be used for BIOS calculations are:
cylinders=96 heads=64 sectors/track=32 (2048 blks/cyl)

fdisk: invalid fdisk partition table found
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 32, size 196576 (95 Meg), flag 80 (active)
beg: cyl 0/ head 1/ sector 1;
end: cyl 95/ head 63/ sector 32
The data for partition 2 is:

The data for partition 3 is:

The data for partition 4 is:


Well, it does say that the partition table is invalid. Now, lets see
what disktype thinks:
root@music1# disktype /dev/da0

--- /dev/da0
Character device, unknown size
Amiga Rigid Disk partition map
Partition 1: 11 MiB (11534336 bytes, 22528 sectors from 4096)
Drive name "IDH0"
Type "NBR\7" (NetBSD root)
UFS file system, 8 KiB offset, big-endian
Last mounted at "/mnt"
Partition 2: 46 MiB (48234496 bytes, 94208 sectors from 26624)
Drive name "IDH1"
Type "NBU\7" (NetBSD other)
UFS file system, 8 KiB offset, big-endian
Last mounted at "/mnt/usr"
Partition 3: 33 MiB (34603008 bytes, 67584 sectors from 120832)
Drive name "IDH2"
Type "NBS\1" (NetBSD swap)
UFS file system, 8 KiB offset, big-endian
Last mounted at "/mnt"
Partition 4: 4 MiB (4194304 bytes, 8192 sectors from 188416)
Drive name "IDH3"
Type "NBU\7" (NetBSD other)
Blank disk/medium

Ok, so this is an Amiga RDB[3] partitioned disk, with NetBSD partitions
on it.
Another zip disk, fdisk again:
root@music1# fdisk /dev/da0
******* Working on device /dev/da0 *******
parameters extracted from in-core disklabel are:
cylinders=96 heads=64 sectors/track=32 (2048 blks/cyl)

parameters to be used for BIOS calculations are:
cylinders=96 heads=64 sectors/track=32 (2048 blks/cyl)

fdisk: invalid fdisk partition table found
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 32, size 196576 (95 Meg), flag 80 (active)
beg: cyl 0/ head 1/ sector 1;
end: cyl 95/ head 63/ sector 32
The data for partition 2 is:

The data for partition 3 is:

The data for partition 4 is:


Hmm, invalid table again. Now disktype:
root@music1# disktype /dev/da0

--- /dev/da0
Character device, unknown size
Apple partition map, 6 entries
Partition 1: 40 KiB (40960 bytes, 80 sectors from 1)
Type "Apple_partition_map"
Partition 2: 5 KiB (5120 bytes, 10 sectors from 81)
Type "Iomega_DiskInfo"
Partition 3: 200 KiB (204800 bytes, 400 sectors from 91)
Type "Apple_Driver"
Partition 4: 95.75 MiB (100406272 bytes, 196106 sectors from 491)
Type "Apple_HFS"
HFS file system
Volume name "System_701"
Volume size 95.74 MiB (100392960 bytes, 65360 blocks of 1536 bytes)
Partition 5: 512 bytes (1 sectors from 196597)
Type "Iomega_backup"
Partition 6: 5 KiB (5120 bytes, 10 sectors from 196598)
Type "Iomega_DiskInfo"

Ok, so this disk has an Apple partition map[5] with a HFS[4] file system
on it.

The final disk, just disktype this time:
root@music1# disktype /dev/da0

--- /dev/da0
Character device, unknown size
Amiga Rigid Disk partition map at sector 2
Partition 1: 11 MiB (11534336 bytes, 22528 sectors from 4096)
Drive name "IDH0"
Type "NBR\7" (NetBSD root)
UFS file system, 8 KiB offset, big-endian
Last mounted at "/mnt"
Partition 2: 46 MiB (48234496 bytes, 94208 sectors from 26624)
Drive name "IDH1"
Type "NBU\7" (NetBSD other)
UFS file system, 8 KiB offset, big-endian
Last mounted at "/mnt/usr"
Partition 3: 27 MiB (28311552 bytes, 55296 sectors from 120832)
Drive name "IDH2"
Type "NBS\1" (NetBSD swap)
UFS file system, 8 KiB offset, big-endian
Last mounted at "/mnt"
Partition 4: 10 MiB (10485760 bytes, 20480 sectors from 176128)
Drive name "IDH3"
Type "DOS\3" (Amiga FFS file system (intl., no dir cache))
Amiga FFS file system (intl., no dir cache)
Type "DOS\3"

Amiga RDB again, the last partition is an Amiga FFS[6] partition.

References:
1) http://www.freshports.org/sysutils/disktype/
2) http://en.wikipedia.org/wiki/Iomega_Zip_drive
3) http://en.wikipedia.org/wiki/Rigid_Disk_Block
4) http://en.wikipedia.org/wiki/Hierarchical_File_System
5) http://en.wikipedia.org/wiki/Apple_partition_map
6) http://en.wikipedia.org/wiki/Amiga_FFS
--
Torfinn Ingolfsen,
Norway