FATAL: Could not read from the boot medium! System halted.

[Syncopator]

When I try to run XP in the virtual box on Windows 8.1, the box screen presents the message which forms this post's Subject.

What, exactly, does it mean?

[Perryg]

It means you have not mounted the install media in the virtual CD/DVD drive.

[ikar.us]

Does the message come from VB, really no boot medium accessible?

Or rather form the installed XP, after the boot process has already begun -
which is confusing, because it obviously could read the boot medium up to this moment.

XP used to do this if hardware changed, e.g. IO-APIC disabled, when it was enabled during install.

[mpack]

It's a standard message from VirtualBox, which means what it says really. The cause is usually one of two things :-

1. That the user confused virtual and physical drives. E.g. the user puts a physical CD in a host drive and expects that somehow the virtual machine will know to look there without being told (fix: configure VM storage settings to map the virtual CD drive to the host drive).
2. The CD provided is not in fact bootable. E.g. it's a service pack CD.