It means Windows can not detect a Partition table on the drive. I would try the easy step first. It involves booting a live linux disk to see if the drive can be detected. Linux can sometimes open a RAW drive where Windows fails.
In order to boot this disk you will need to disable Secure Boot in BIOS and also Quick/fastboot if there is a setting for that. If the disk does not boot with SecureBoot disabled then you will need to enable something called Legacy/CSM boot. What is the make and model of the computer?
Download Fatdog64 here. You can burn a disk by right clicking the iso file and selecting Burn Disk Image. You can also create a bootable flash drive with the iso and Rufus. If you use Rufus select MBR for a partition scheme in the dropdown box. Where you see Freedos select iso image. Click on the folder icon and browse to the Fatdog iso file. Press Start. If any data is on the flash drive back it up first as the flash drive will be formatted. Make sure your flash drive is detected in the first dropdown box.
Boot Fatdog. In the lower left desktop you will see your hard drive(s). You Windows drive will be labled sda with numbered suffixes. These numbers represent the partitions on the disk. Your external drive if connected internally may be called sdb or sdc. Clicking once on the drive icon will automatically mount the drive and open a file manager if it can read the RAW drive. If it does then you can copy the files. Post back if it does and I will post instructions on how to copy your files. If not then we will need to try a different program that scans the drive to find the partition table.
Edit: What is the size of this drive?
Edited by JohnC_21, 21 November 2015 - 12:09 PM.