The most common reason for this problem is that the drive has been prepared
with special partitioning.  Now that it is a slave, the driver software
required to access the partitions is not being loaded on boot, and the drive
appears to be unpartitioned.  Windows will not assign a drive letter, and
you can't access it.

Special partitioning such as EZ-BIOS or Disk Manager might have been
required if a previous machine where the drive was used could not see the
full capacity of the drive with the installed BIOS.  Another possibility is
that the drive was prepared with special management software such as GoBack.

Can you access the drive if it is configured as master?  This will confirm
that the problem is the loading of the management software.

You can usually get access to the drive by installing the manager software.
It should be possible to install the software without converting the
existing master drive to use it.  However, my preference would be to put the
drive back where it came from, copy off the files you need, then rebuild the
drive in the new machine with standard partitioning.