Hi DF94. Welcome to the forum.
I haven't installed a dual boot system in a long time, but the good thing is that I remember that dual boot with Windows 98 it was easier than with Windows 8 or 10.
As Laurence suggest, It is better to use AirBoot (included on ArcaOS) instead of the old IBM Boot Manager. AirBoot installs on the first partition of the hard drive.
I think that the trick is to take a moment to define the partitions layout on your hard drive.
For example I made this VM with one virtual HDD to try to install ArcaOS and Win98.
This is a VHDD with 20GB and I organized this way:
- 1 - Windows 98 - 5GB - Primary
- 2 - ArcaOS Partition - 5GB - Primary.
- 3 - Shared Partition - 10GB - Logical
I think that Windows 98 will not let you install if it is on a second partition away from the first sector/part of the HDD. (I don't remember the name of that limitation).
I did this layout with ArcaOS tool for partition the HDD on the installer. No extra tools.
I also asked to install the "Boot Manager" (which is Airboot) from the same tool.
After that I installed ArcaOS 5.0 Normally on the second partition. ArcaOS names this partition C:
Once ArcaOS finished instaling, you will see at book AirBoot first and them it will boot ArcaOS.
Now, let's install Windows 98. Boot the CD (an ISO image on this VM) and let's see if it accepts our VHDD layout.
Windows 98 is not a nice player, it will ask you to remove any other OS, JUST SAY NO!!!
Win98 will not see the ArcaOS partition and will think C:\ is the first partition and will ask you to format it. Windows will format it as FAT32. Here I did some trick, since Windows 98 seem that didn't like how ArcaOS made the first partition, I pressed "F3" to go to Windows98 command line and run FDISK. I deleted the first partition and recreated it with FDISK.
After that I was able to normally install Windows 98.
Once Windows 98 is installed and running, it would break AirBoot, so you can not get back to ArcaOS. To fix this you need to boot again with the ArcaOS CD and install "Boot Manager" again.
Now it seems to work for me on my VM.
Maybe it could be easier if you only make the HDD layout with ArcaOS, install Windows 98 and then install ArcaOS.