os-prober and update-grub can't see/find working Windows install

I have had the same problem:
Windows 7 and Linux (Slax with Grub2 and Debian in my case) and os-prober didn't recognizes the Windows partition when I used update-grub; Windows was left out.

I repaired the problem. First I tested adding the entry for Windows 7 manually to /boot/grub/grub.cfg (update-grub overwrites this file whenever it is run, so changes made to it are not permanent).

I rebooted and Windows 7 appeared in the menu and booted ok, aside from this error appearing:

error: no suck device: xxxxxxxxxxx (disk ID)

When this came up, I pressed Enter and Windows booted fine.

I investigated the execution of the os-prober command and others scripts involved in the process of recognizing OS and I test the output of

sudo /etc/grub.d/30_os-prober 2>/dev/null

Windows is not listed. I opened Dolphin file manager to browse the Windows partition and searched for the UUID of the partition and replaced it in /boot/grub/grub.cfg to resolve the error.

I executed

sudo /etc/grub.d/30_os-prober 2>/dev/null

and Windows was then listed ok

I rebooted the PC, went to console, and executed

sudo /etc/grub.d/30_os-prober 2>/dev/null

and then Windows was again not listed.

I opened Dolphin, browsed the Windows partition, executed

sudo /etc/grub.d/30_os-prober 2>/dev/null

again and Windows was listed.

I executed update-grub and the GRUB menu is now ok.

I think os-prober needs the partition (in NTFS case, because the Debian partition always is listed) mounted before it can find it. I haven't found out why os-prober has this behaviour but the solution works.