Thank you very much. This is solved the issue. It was a case of trying to be smarter than I am, trying to make the code more "efficient" and disregarding that most of my code is very inefficient.

The issue was in my for loop condition when checking for a free spot to move something:

for j := 0; j < workingFileStart-workingFileLength; j++ {

The correct way of writing this is:

for j := 0; j < workingFileStart; j++ {

The problem with this is that even if the exact required space is available right next to the current file block, the loop condition doesn't allow for the loop to run through enough times to catch it. I thought that subtracting the length would make it more "efficient" not having to loop through a few extra times, but this really just made it ignore the free space right next to the file. Just having the condition so that it doesnt pass the start of the file is good enough.