Try booting from a DOS/Windows 95 boot disk (make sure the boot disk you use matches the OS on the drive you want to boot from) and run FDISK /MBR (It won't say anything in response.)
If that still doesn't work, reboot from the boot disk and try SYS C:
If even that doesn't work, you'll have to re-partition the disk using FDISK on the system you want to use it in (meaning, don't put that drive back on the 486 to do it), and format it afterward. (Afterward, you can copy all the files to it from a different computer.)
Sometimes the BIOS doesn't like the disk's boot sector if the disk was prepared/used in a different machine prior. I've seen this happen many, many times, and usually I am able to work around it via FDISK /MBR or SYS C:.
Life isn't long enough to re-enable every hidden option in every BIOS on every board... 🙁