Macrium fails only during shutdown

I am having problems with Macrium Reflect Standard (purchased) edition.

I have built a new PC for one of my daughters family and have setup Macrium to automatically backup when the PC is shutdown. With all the software loaded it works very nicely backing up to the backup disk.

Then I copied over some family pictures and videos, about 50gb is size.

Before I added the data the system backed up in about 4 minutes perfectly. With the extra data the backup should take about 20 minutes but it is being terminated at 14 minutes approx (about 62% complete) when it runs during shutdown. If I run it from the desktop icon it completes properly, and it also runs correctly if I tell the PC to restart. Its only during shutdown that it fails.
The error that flashes on the screen is error 21, read failure, I get this twice.

Software is win7 professional patched up to date, the hardware is mirrored drives on an Adaptec 1220 raid card, and the backup disk is on a standard sata port.
The problem is not the raid card as I tried a RocketRaid card with the same results.

I have turned off the group policy timeout for allowing scripts to run but this has not had any effect.

There is obviously another timeout in windows set at 15 minutes or so which needs to be changed, but I cannot find it.

Problem is now resolved. There two timeouts to change, one at 10 minutes and another at 15 minutes.

If interested look at the Macrium forun on their website. All details are there.