There are several possible scenarios going on here. The good news is, most of them are completely benign and relatively easy to fix.
The bad news is, the one that’s not benign is pretty serious — as in, “I hope you have a backup” serious.
Most common: USB boot
These days, the most common reason for this error is that:
The solution is simple: unplug the USB flash or external hard drive, and try again. If your machine now boots, you know this was the problem.
Now, your machine isn’t supposed to boot from an unbootable USB device. It should notice and move on to your hard disk, even if there’s a USB disk attached. Most machines will do this, but I’ve seen machines where, if there’s any disk attached, and the boot order puts USB in front of the hard disk, an “invalid system disk” error results.
The only long-term solution in a case like this is to change the boot order in your BIOS (or UEFI) so the hard disk is checked first. Unfortunately, I can’t give you step-by-step for that, because it differs for every machine.
Less common: CD boot
This is the same scenario as with the USB disk, but involving the CD or DVD drive instead.
- Your machine is configured to look for optical devices (CD/DVD) from which to boot before it checks the hard disk.
- You have a CD or DVD inserted.
- That inserted disk or drive is not bootable.
- Your computer is trying to boot from it anyway.
Your options are similar to those above: don’t leave an optical disk inserted when you reboot, or change the boot order of your BIOS.
Blast from the past: floppy boot
Anyone who has used a computer since the days of the floppy disk has probably already seen this error. Machines with floppy drives were configured to — you guessed it — boot from the floppy disk if one was inserted. If you happened to have the wrong disk in there — one that was not bootable — this message would result.
Been there, done that.
And again, same options as before: eject the floppy, or change the boot order.
Hope you have a backup: malware or other software
Your computer’s BIOS or UEFI looks for specific information on your hard disk to determine if there’s an operating system stored on it. If that information is erased or damaged, then as far as the BIOS is concerned, there’s no operating system to load.
It’s not common these days, but I do recall malware that would do this. It’s also possible, though extremely unlikely, that a simple crash could cause important portions of the disk to be overwritten in error, erasing the boot information.
While there’s often some hope for data recovery if you don’t have a backup, the real solution is to restore your hard disk from your most recently created image backup. (Ironically, that’ll probably mean booting from a rescue CD or USB drive you created using your backup software.)
REALLY hope you have a backup: hard disk failure
This error can also be caused by hardware failure.
Disks do fail. Occasionally, the circuitry that connects to the hard disk fails, or something else in your computer fails in such a way that your hard disk is prevented from operating properly.
If the drive has truly failed, there’s rarely hope for recovery of the disk itself. This is a case — perhaps the case we keep warning about — where you must repair or replace the broken hardware, and restore your data from your most recently created image backup.