NTFS HD compression

HI

I got an exchange server with 18G HD that split into two equal partitions of 9 gig each. priv.edb (7.5 gig) is on the second partition and is compress.
I want to do an offline backup by copy the file onto the network but it keep giveme message “can not copy need to delete other file”.
I had never seen this problem with file copy. Second partition has 1 gig of free space left.
I will get HD but I need to be able to backup copy first. If I stick on 9 gig hd will it let me copy?