virtualbox.org

Host: Linux CentOS 5.4 32 Bit
Guest: Windows XP SP3 32 Bit

This is more of an informational post on how i resolved booting a windows guest that wouldn't start up after my host system crashed. Host crashed because i had too many apps/guests running and ran out of memory.

After rebooting the host and starting Virtualbox, i tried starting a Windows XP guest and didn't get anything but a black screen. No error message, no activity on the disk/network icons, nothing but a black screen that just sat there. I tried another Windows guest and it started without a problem. Both were running when my host happened to crash. I thought it might be vt extensions or something keeping it from booting so i disabled everything on the guest that wasn't needed to boot (network adapter, sound, vt extensions, etc. When this didn't work i tried creating a new guest but instead of creating a new disk, used the windows xp disk from the broken guest. Still nothing but a black screen. Looked in the forums and googling for similar issues but couldn't find a fix. At this point i figured the system might be hosed and there might not be a chance to recover. The next thing to try was adding this broken disk as a second drive in my other windows guest. Went into settings in my working windows guest, added a new ide controller, and then attached the hard disk from my broken guest. When i booted, Windows checkdisk started running and wanted to check the new drive. I let this run and it did find some files that needed to be repaired. When the system finished booting i was happy to see a new d: drive and all of my files. At least i knew i could recover this way. I thought it was worth another shot so i shutdown the virtual guest, removed the ide controller and drive and then tried starting the broken guest again. This time it booted right up without any errors.

Now i do have one question. Since it seems the guest disk was corrupted by the host crash, does virtualbox have a tool for diagnosing guest vdi files and reparing them?

You just found the tools. a VM is a harddisk, any normal dianostic tool will work.

The problem is that its' not a hard disk until it's mounted as a hard disk. Until then, it's just another file on my linux host. When the vdi is corrupt and Virtualbox won't load it, how do you mount the vdi to check it? Yes, i see now that there are 3rd party tools to mount vdi files (if they are fixed size vdi files, mine was dynamic) but seems like Virtualbox should have this built in. Are you implying that i can fsck my host linux filesystem and it will fix ntfs filesystem errors inside my vdi files? The workaround was the only way i could get things working again and that's why i posted it. Is there a better way, something i'm missing?

Edit:
One thing that i forgot to mention was that i did try booting from the windows installation disk and it couldn't find the hard disk. Looking back, the reason it couldn't find the disk is because i had changed the drive from ide to sata under virtualbox and the windows xp cd didn't have the correct drivers to load a sata disk. What i should have done was change back to ide in storage details for the guest and then run the windows install cd repair program. This would be the correct way rather than the workaround i posted just in case someone else runs into this problem.

Can you attach the VDI to another VM as second disk?

Brilliant! Thank you - I was really worried. Had exactly same symptoms as you - Windows XP guest - blank screen, no cursor, no activity. Would boot to recovery mode from ISO and interrogate disk fine. Never occurred to me to run chkdsk - I thought the problem was between VirtualBox and the VDI.
My sequence of events was:
Was running guest in background
Started another program (PDFEdit) in host (Ubuntu 10.10) which after adding several pages started to leak memory. Paused VM to release more processor time (here I should have shut it down to release memory but hindsight is 20:20). Watched everything come to a grinding halt (was under a deadline so had to hard reset).
On starting VB after reboot, got blank screen. Tried pausing and resuming in case it was because it had reset while paused. Tried changing the controller (I use SATA as it's faster) to IDE. No deal - panicked. Looked online and found nothing... until this post. Restarted with XP ISO, ran chkdsk from recovery console...
Worked like a charm!
Thank you so much - without your post I'm not sure I would have fixed it.

Duncan Gallimore