The BeOS install CD is not a standard ISO, it has several tracks using different filesystems and needs a little adjustment to make VirtualBox happy.

After extracting the BeOS 5.0 Professional.7z file you should have three files:

• BeOS_5.0_Professional.ccd
• BeOS_5.0_Professional.img
• BeOS_5.0_Professional.sub

These are from a program called CloneCD which reads the raw CD data and then saves it in its own CCD image format. The img file is the raw data and the ccd file contains information about where the various tracks are (we don't need the sub file for what we're doing). We need to convert this to a format that VirtualBox understands: cue/bin.

Luckily there's an open-source utility from GNU that can help us with that: ccd2cue. This will convert the ccd file into a cue file. If you'd like to compile it yourself and run it you're welcome to do that, but the file it generates is small enough to copy and paste:

FILE "BeOS_5.0_Professional.img" BINARY
  TRACK 1 MODE1/2352
    INDEX 01 00:00:00
  TRACK 2 MODE1/2352
    INDEX 01 07:48:09
  TRACK 3 MODE1/2352
    INDEX 01 40:51:64

FILE "BeOS_5.0_Professional.img" BINARY
  TRACK 1 MODE1/2352
    INDEX 01 00:00:00
  TRACK 2 MODE1/2352
    INDEX 01 07:48:09
  TRACK 3 MODE1/2352
    INDEX 01 40:51:64

Save that as BeOS_5.0_Professional.cue.

Now you can go back to the VM settings, Storage, click the CD drive, click the little CD icon next to IDE Secondary Master, Choose Virtual Optical Disk File, and find your new cue file.

If everything went well, it should look like this:

Click OK and let's install!