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I've installed Ubuntu 12.10, then I made a backup from root.disk file. I decided to change from Ubuntu to Xubuntu. So I downloaded Xubuntu 12.10 ISO image and reinstalled Wubi.

In case I'm want to rollback to Ubuntu, is it safe to copy root.disk to C:\ubuntu\disks without reinstalling Wubi first to Ubuntu?

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Yes, it's safe as long as they're the same release (which they are in this case) and you installed to the same partition (C:).

When you boot your computer it will still offer a choice between Windows and Xubuntu but it will boot whatever is on the root.disk. If you wanted to change the boot entry back to Ubuntu you'd have to use bcdedit or easyBCD (for Windows 8/7/Vista) or edit the boot.ini (Windows XP).

Wubi boots via grub4dos (wubildr.mbr) and a version of grub2 in wubildr. What they do is locate the first /ubuntu/disks/root.disk it finds on any partition, and then load the Grub menu (grub.cfg) from within that root.disk. So it doesn't matter what is on that root.disk (or even what partition, provided in the grub.cfg it has the correct partition UUID). So if you installed on C: it has to be on C: or you have to manually edit the Grub menu entry before booting.

The only issue is that the modules built into wubildr must be compatible with the grub.cfg so changing releases can lead to incompatibilities. In these cases you can sometimes manually boot from a grub prompt, but it's best to stick to the same one to avoid issues.

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It works, because I used it from same releases –  Aryo Adhi May 21 '13 at 5:18
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