From PM Help:

#105 Partition starts on wrong boundary
The hard-disk partition table contains erroneous values. PartitionMagic expects
partitions to begin and end on the correct cylinder boundaries. If they do not, the
disk may be partially corrupted. In this circumstance, if PartitionMagic were to
make any modifications it might cause the loss of data. Therefore, PartitionMagic
refuses to recognize any of the hard disk’s partitions. To resolve this problem, see
the instructions in “Resolving Partition Table Errors” on page 117:

Resolving Partition Table Errors
Partition table errors are errors in the 100 - 199 range. In most cases, you must resolve
partition table errors by creating new, error-free partition tables. The general steps are:
(1) ensure you have no viruses (see below),
(2) back up the data on the affected partitions,
(3) delete the partitions,
(4) recreate them, and
(5) restore their contents.
You may need to use the FDISK program from a recent DOS version, as earlier versions may refuse to delete
HPFS or hidden partitions, and the OS/2 FDISK program may recognize the partition’s
corruption and refuse to modify it.
In some cases, you can resolve partition table errors manually. Run PartitionInfo to
determine the errors on your partitions. PowerQuest technical support can help resolve
partition table errors if you e-mail the PartitionInfo report to Diese E-Mail Adresse ist gegen Spam Bots geschützt, Sie müssen Javascript aktivieren, damit Sie sie sehen können
Refer to “Generating Diagnostic Reports with PartitionInfo” on page 118 for additional
information about PartitionInfo.

It is possible to force partition magic to read the partitions by using the switch /ipe
pqmagic /ipe
This way you don't have to use Fdisk to reformat the drive.