Error backing up data; dual boot related?

First, apologies if this is the wrong forum for this; Backup & Recovery seems like Maintenance, so I put it here.

I have been using the built in Windows 7 backup feature for several months without any problem. My last successful backup was on March 6th. The last attempted backup on March 13th (set on weekly schedule) failed for the following reason: "The volume for a file has been externally altered so that the opened file is no longer valid." The details show an error code of 0x800703EE. Every attempt to manually back up fails within a minute for this reason.

My backups all go to a big external hard drive connected by USB. I haven't had any problems like this in the past. Searching on Google reveals very little concrete info about what this means or how to fix it, although quite a few people seem to have come across it. The most informative post on the subject was this, which has the following solution:

Issue is resolved. The root cause was error in bcd, bcdedit was giving "The volume for a file has been externally altered so that the opened file is no longer valid" when trying to /enumerate active. Fixed it manually by building new BCD and doing a import followed by reboot. I guess windows backup internally tries to enumerate BCD and fails without correct log/message when enumeration is not successful.
Backup is running fine now.

I think BCD refers to "Boot Configuration Data," which worries me. The major significant change between my last backup and now is that I shrank and moved around partitions in order to make room for an Ubuntu 10.10 install, which currently resides on a logical partition in the same extended partition as my general purpose Data partition. The Ubuntu install replaced whatever bootloader the laptop initially had (which I backed up) with GRUB. I fully intend to use this laptop as dual-boot Win7/Linux box, so "rebuilding" a boot config seems like it would prevent me from doing this normally. I don't see why this should prevent me from running backups, and really don't want to have to use a tool like Cobian with its torturous interface. Any kind of input or advice on this would be appreciated.

Will reproduce specs from my profile below:

System Manufacturer/Model Number
Asus K50ID Notebook
OS
Windows 7 Home Premium 64 bit
CPU
Pentium Dual-Core T4400
Motherboard
?
Memory
4 GB
Graphics Card(s)
NVIDIA GeForce GT 320M
Sound Card
Integrated
Monitor(s) Displays
15.6" laptop screen
Screen Resolution
N/A
Hard Drives
1x 320 GB internal (Samsung HM321HI SCSI) with a 17 GB recovery partition, 75 GB Windows 7 system partition, 192 GB data partition, and 36 GB Ubuntu 10.10 partition.
1x 500 GB external (Western Digital 5000AAK "MyBook") connected by USB.
PSU
?
Case
N/A
Cooling
?
Keyboard
N/A
Mouse
N/A
Internet Speed
N/A
Other Info
N/A

So... is this basically a mystery?

I have independently come upon what may be a fix. I had a related problem with hibernation after setting up dual boot that required me to set the Windows OS partition to active. After that, I tried backing up, and it created a brand new backup... the old ones are gone, but that's OK.

This might be a fix for this issue. Just putting it out there in case someone runs across this topic while looking for a solution to this problem.

Grub is known to corrupt Win7 when on the same HD. It is always best to install to another HD booted via BIOS Boot Order or one-time BIOS Boot Menu key.