[ubuntu] Stuck in GRUB Rescue

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http://ubuntuforums.org – I have a problem that I have not been able to find an answer for after searching. I have a PC where I can swap out the system disks (I use those slide in/out SATA trays). On one disk I installed Win7 - everything ran fine. I removed the first disk and inserted a second disk and installed Ubuntu - that ran fine. Now here is where the problem starts: I removed the second disk (Ubuntu) and insert the first disk (Win7), when I try to boot up I get the following error message: "no such device grub rescue" If I re-insert the second disk (Ubuntu) it will boot into Ubuntu ju (Hardware)