I dual boot Windows 7 and Windows 8.1 (upgraded from W8) and use the System Reserved partition. After upgrading to Windows 8.1, I opened Macrium Free 5.2 which updated to ver 6427 and when I tried to image the System Reserved partition it failed with this error:

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I booted to Windows 7 and it worked OK. After visiting the Macrium site I found the problem. During the upgrade to 8.1 the System Reserved partition was assigned a drive letter. After removing the drive letter Macrium worked OK.

Just a heads up to other users.