This question was asked by a user on Microsoft Answers website las year, but it had no answers so far (unsurprisingly). I am facing the exact same issue.
Here are the steps to reproduce this problem using the same computer :
Install Windows 7 and use additional hard drives with data on them. In my case they used to be on an XP/Vista system. There will be no problems.
Next, use the hard drives back with XP/Vista. There will be no problems.
Now, use the hard drives again with Windows 7 and it will install automatically device drivers for these hard drives requiring restart. After the restart the drives will not be available in My Computer and the Disk Manager will show the entire drives with unallocated disk space.
Use the hard drives back with Vista and they will be fine with the data showing again.
What I don't understand is how Windows decides that certain hard drives need new device drivers. How come my drives worked perfectly right after the installation with Windows 7 but once used with Vista, then back with Windows 7 they got automatically new device drivers. And why installing device dirvers has to render the disks as unallocated space when they already have perfectly healthy working data with previous Windows systems.
This makes dual boot of almost no use when using multiple HDD systems, and such a flaw is simply unacceptable. To make a long story short, how to make W7 recognize my HDDs with the data in it AFTER I used them on an XP/Vista OS.
Original Question/Issue here