Sorry.
Well, ok, SRC means Source, or where the files are gettign moved FROM, and DEST being where they are going to...
For example if you have C: and D: then obviously the files from D: are being moved to C: and therefore D: will be the source or SRC and C: will be the DESTination...
Well, in Windows, move what you can from D: to C: until there is no more room.
Then in Acronis, shrink D: and expand C:
Do this again if need be.
This only works if there is very little room on both.
The thing is, that merging the partitions does not actually merge them at all... It merely makes the D: Partition into a folder on C:
C: and D: will still be seperate partitions.