I was trying to upgrade a few 2008 R2 servers to SP1. Two of the three were successful without any issues. The third gave me some trouble – so I found a way to fix it.
Fast forward a couple more weeks and I upgraded a pair of identical 2008 R2’s to SP1. One success. One Failure. So now here’s my documentation (lot of help from http://jasonduffett.net/post/5189439239/windows-2008-r2-service-pack-1-error)

Download and run this file:
http://www.microsoft.com/downloads/en/details.aspx?FamilyId=c4b0f52c-d0e4-4c18-aa4b-93a477456336&displaylang=en

Open the log file:
C:\windows\logs\cbs\checksur.txt
Scroll to the bottom where it says “Unavailable repair files”
We want to know the servicing\packages\PACKAGENAME

Take ownership
takeown /f c:\windows\servicing\packages\PACKAGENAME.cat
takeown /f c:\windows\servicing\packages\PACKAGENAME.mum

Grant permissions
icacls c:\windows\servicing\packages\PACKAGENAME.cat /grant administrators:F
icacls c:\windows\servicing\packages\PACKAGENAME.mum /grant administrators:F

Copy from another system
copy \\anothersystem\c$\windows\servicing\packages\PACKAGENAME.cat "c:\windows\servicing\packages\"
copy \\anothersystem\c$\windows\servicing\packages\PACKAGENAME.mum "c:\windows\servicing\packages\"

Re-run the assessment installer tool.
Re-run the sp1 installation.
Profit