Warning for Not Enough Disk Space misleading ?

[TNT@FFS]

Hi all,
although being a happy long term user of FFS by now I am new here in the forum, because the first time I ran into something that I can't get my head around. I found some older information form 2022 referring to disk space issues, but it didn't answer the question.

Part of my backup strategy is to mirror volumes used for data archives to one or two other volumes for external storage, in this case an offline media archive stored on a 4TB WD Red formatted in exFAT was to be mirrored to a second disk of the same type.

When trying to start the mirroring process I got the warning that there is not enough space on the destination drive. At this time the source drive reported about 65GB free space and the destination drive about 25 GB, while there were about 160GB to be mirrored. So at first glance the warning seems to be correct. However, most of these 160GB were files to be updated on the destination drive rather new data to be added.

My simple question is, whether the rule behind the warning might incorrectly refer to the total amount of data to be transferred (i.e. new files as well as files to be updated) rather than refering to the actual quantity of new data only.

In fact, after ignoring the warning the mirroring process was completed without errors or further warnings. Afterwards the source disk reported about 43GB of free disk space, the destination drive 47GB, although after the comparison there were no files marked as to be deleted on the destination drive.

Any ideas / explanations out there ?

Thanks in advance for your help (P.S. FFS version is 14.5 run under MacOS 15.7.1)

[Plerry]

> My simple question is, whether the rule behind the warning might incorrectly refer to the total amount of data to be transferred (i.e. new files as well as files to be updated) rather than referring to the actual quantity of new data only.

When updating files (=replace the existing/previous copy by a modified copy), FreeFileSync (FFS) retains the existing/previous copy until the process of copying the modified copy has completed. Only upon successful completion of the copying process of a file, the existing/previous copy is deleted.
Conversely, if the copying process fails for whatever reason, the existing/previous copy is restored, such that at least that existing/previous data is not lost.
So, temporarily, both the existing/previous copy and the modified copy of one or more files will exist in your destination location.
Depending on your FFS edition, file copying may occur in parallel threads, so in extremis all file copying could occur simultaneously, temporarily requiring double the space.