Re: Grub rescue, usb boot no longer possible
When you boot from USB flash memory it takes much longer to boot, so are you sure that you are giving it enough time? Depending upon what you are using, it may take over a minute.
Were you using Wubi to run Ubuntu withing Windows or did you actually have a separate Linux partition?
Often a different partition boots Windows than you think, and even if you restore the mbr to a suitable Windows mbr or using an mbr from syslinux (which does the same thing) you need to have the proper partition marked as boot for Windows to boot, which may be a different partition than you think (especially for Win7).
For example when I tried Ubuntu on a Dell laptop that I was setting up for someone I used Win7 to shrink its own partition and put Ubuntu with grub on sda4 with sda4 marked as boot partition (leaving Windows mbr intact), so I thought Ubuntu would be easy to remove. However, I neglected to do an sudo fdisk -l before installing Ubuntu, so when I removed sda4 and marked the Win7 partition as boot, Win7 would not boot until I repaired it several times with a Win7 system disk.
When I bought a Dell desktop I discovered that Windows was actually booting from sda2 (RECOVERY) instead of sda3 (OS). I currently have 64-bit 10.10 and its grub on sda4 and 11.10 on sdb1, and boot everything from 11.10's grub on sdb. That is because some Dell program (I think Dell Datasafe) steps on grub2 saving data in what it "thinks" is an unused part of the sda mbr.
i5 650 3.2 GHz, 8 GB, nvidia GTX 550 Ti 32" 1080p | i7-4700, 8 GB, Intel HD 4600/nvidia GTX 765M 15.6" 1080p | etc.