VHD boot problems are rare, when following exactly the Tutorials linked in this Post Signature. Most are caused by a corrupted BCD (Boot Configuration Datastore) or missing boot-critical disk & controller drivers, enabling reading the VHD or hard disk where its saved. If they occur, follow the troubleshooting steps for natively supported & unsupported OS boot from VHD:
1) install latest Windows Virtual PC (with 1024Mb+ RAM assigned), attach Win7 VHD to it as IDE disk and try to boot as hard drive. If it doesn't, it may be because of missing or errors in BCD, missing Bootmgr, mismatch of disk Sig, or incorrect partition offset set in BCD, DetectHAL element is Off or missing, missing disk driver, or volume wasn't marked Active. This may occur, since Win7 & ThinPC may lack BCD sample store in their WindowsBoot folder, resulting in wrong BCD structure when using BCDBoot command to add BCD to the new VHD. To fix, use suggested repair sequence. However, if you face 0x7B error during boot from VHD, you may need to add missing disk driver(s) entries to VHD's OS Registry, and add the driver installation packages, so that OS can autoinstall the drivers after boot. Read this and that explanation threads, and use suitable tools, like included with IMG_XP driver utilities and its collection of Registry fixes. Or, once you found missing driver INF packages, attach the VHD, mount its OS Volume to an empty folder and inject the drivers offline with DISM as per Install Unsupported OS to VHD Tutorial. If the VHD boots after that, proceed to step 2.
2) copy that Win7 VHD from HD to a bigger fixed contiguous VHD, add boot environment to the bigger VHD disk with EasyBCD or Visual BCD Editor installed on your system HD, add Win7 boot entry to it, attach the big VHD to Win VPC as IDE disk, try to boot Win7 from VHD natively inside VPC. If it doesn't, fix the big VHD's BCD using the below repair sequence. After it does, proceed to step 3, if you want test booting with Grub4DOS & WinVBlock.
3) add Grub4DOS to the big VHD, fully install WinVBlock to Win7 after booting it natively from VHD or as IDE disk inside VPC, make Win7 VHD contiguous, attach the big VHD to the Win VPC as IDE disk, try to boot Win7 VHD from it via G4D & WinVBlock menu. If errors, try to find matching versions of Grub4DOS & WinVBlock (older G4D may work well with WinVBlock - Feb. 2011, can take from wimb's IMG_XP folder). If Grub4DOS shows "File not found" error on boot, and the file name is correct in its Menu, the problem seems to be in mounting the large VHD disk. You must reboot G4D to continue tests or restore original disk mapping. Make sure to test sign WinVBlock 64-bit, and switch On Test Mode in Win7 64-bit, or use F8 at boot. Should boot... but at times WinVBlock disk may cause OS instability & file access issues after boot.
Boot troubleshooting requires some practice, so easy way to start - native boot VHDs & EasyBCD. Your findings reports are useful and appreciated, make them detail, don't miss small things.
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