Probably has some sort of disk overlay software on it.

Two things:

First, yeah, depending on the age of the computer, it may not have hardware-level support for larger hard drives (>512MB). This requires the so-called overlay software to translate DOS I/O calls and bypass BIOS Cylinders-Heads-Sectors limitations (typically 1024 cyl, 16 heads, 63 sectors) allowing you to access the full capacity of the disk. The overlay driver gets loaded when booting from the disk, so booting from floppy or CD means you're back to the limited BIOS API, and will only see the first 1K cylinders. Plus, the virtual geometry the BIOS sees is likely to be different from what the overlay uses (since only the oldest drives actually told the truth about how many platters and heads they had), so the partitions and file systems will appear to be corrupted. You can think of it kind of like the disk is encrypted, and the decryption is done on-the-fly by the driver that loads before DOS when booting from the HDD.

Second... most modern IDE interfaces have a really hard time with old hard drives. So, the last few generations of computers that had IDE interfaces may or may not be able to read old drives (less than, say, 5 to 10GB.) None of the USB-to-IDE interfaces I have work right for those drives. In Linux, it'll mistakenly think it's a really large hard drive, which it most definitely is not, and then fail to actually read from the disk at all most of the time. In Windows, it probably just throws an I/O error (but I haven't tried it.)

To fix this, you need a boot disk with support for whatever drive overlay software your disk is using (possibly On-Track Disk Manager? -- I think that's what WD used back then), and a real IDE interface on a computer that will support a 720MB HDD. Or, you can backup the contents via network, serial/parallel cables, floppies, CD, ZIP, Compact Flash/SD, etc.., then use fdisk /mbr to erase the disk overlay software, and rebuild using a standard DOS layout. You might not be able to access the full size of the drive if your Gateway's BIOS doesn't support cylinders >1024 though.

thanks. thats the most useful info i have seen and ive searched the internet for hours. im getting my hands on some floppy disk tomorrow and im going to wipe the drive and start fresh.

Glad it helped. This kind of info is falling into obscurity, as by the time any average joe had Internet access, we were pretty much over the whole 512MB BIOS limit kerfuffle. 😉 "Google knows all" .... but only past about 2005 or so.