how to find out what's mysteriously eating HD space (file sync)

[brotherS]

Hello,

I have two external HDs (same model) and mirror everything from #1 to #2 with a file synchronizer tool every now and then. But Windows Explorer now tells me that there are about 5.8 GB more on #2. Huh? 

I then used Beyond Compare to compare both drives, but wasn't able to find the difference. Everything looks the same.

What can cause this? I never saw this in the past.

[wraith808]

Have you run scandisk or some other disk diagnostics?

[skwire]

Running a chkdsk sometimes results in it finding the sort of slack space you describe.

[wraith808]

Running a chkdsk sometimes results in it finding the sort of slack space you describe.

Yeah, that's what I meant when I said scandisk 

[MilesAhead]

I have two external HDs (same model) and mirror everything from #1 to #2 with a file synchronizer tool

Just to be sure, I take it these two drives are spinners?  I know sometimes SSDs will show deleted files as unavailable until the memory can  be "reconditioned" or whatever they call it, so that writes are fast again in those areas.

[Deozaan]

You may also be able to use some tool like WinDirStat to see what's taking up space on your drives.

[Target]

if its only a mirror, then why not just wipe it and re-synch?

if it re-occurs then you know you've got a problem

[Stoic Joker]

Could File History (fka Previous Versions) be using one of the drives?

[brotherS]

I tried chkdsk but it didn't show anything not normal.

Yes, those are HDDs.

> if its only a mirror, then why not just wipe it and re-synch?

Because it's >1 TB of data and both drives are connected only by USB 2.0. 

No "File History" in use here.

WinDirStat finally helped me find the issue: a file with a >4 GB size only had a "size on disk" of a few MB on one drive but >4 GB size on the other. So weird! A similar finding with a >1 GB file. Now both drives have the same available disk space again! The file synchronizer might have an issue with larger files, I'll keep an eye on this.

Thanks for all your help! 

[MilesAhead]

The file synchronizer might have an issue with larger files, I'll keep an eye on this.

That could be it.  I was using a dedicated folder sync program.  It worked fine until it hung one day.  I assume the number of files, or the number of files over a GB, got to be too much for it.  I switched to the folder sync in FreeCommanderXE and so far haven't had an issue with the same folder set.

[Deozaan]

The possibilities that spring to my mind are:

1. Are both HDDs formatted using the same filesystem? (Does that filesystem support large files?)
2. Do you have "Compress this drive to save disk space" enabled on one or both drives?

I recently-ish discovered that enabling drive compression kept causing my source code repositories to become corrupted. The problem plagued me for over a year and I had to keep pulling a fresh copy of my repo from BitBucket because my local copy kept getting corrupted. I even bought a new HDD because I thought the corruption was a sign that my old one was failing. So now, with multi-terabyte drives becoming ubiquitous and relatively inexpensive, I would recommend to keep this option disabled to avoid rare problems like what I experienced.

[brotherS]

@MilesAhead: ah, good to know.

@Deozaan: yes, both are the same. No, never was a fan of that.