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My secondary 1TB internal hard drive filled up recently, so I bought a 2TB disk to replace it. The cheapest 2TB disk I could find was actually a Buffalo Drivestation USB3 enclosure. I used SyncToy to copy the data from the 1TB disk to the 2TB disk, then swapped them around so that the 1TB disk is now in the external enclosure.

The thing is that the 1TB disk is now showing 50GB free space that wasn't there when the drive was mounted internally. Where did this space come from?

(The extra space is about the size of the offline files cache I deleted a few months ago, but I suspect that's just co-incidence, particularly as I got that space back when I deleted those files - although Windows did not make this easy.)

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1. Are you the only one that uses that drive / computer? 2. If you mount it internally again does it show that extra space? – medigeek Oct 12 '13 at 11:45
I am the only one who uses this drive. It does contain backups of at least one previous Windows install. The enclosure is tricky to get into, so I'm unlikely to try mounting it internally again. – Christi Oct 12 '13 at 12:18
It's been a while since I've used SyncToy so pardon my ignorance, but could it be that it automatically ignored any duplicates, thus the sudden free space? – happy_soil Oct 12 '13 at 12:19
No, because the 1TB drive is the source drive, not the destination. – Christi Oct 12 '13 at 12:21
@medigeek Almost. The old drive appears to have more free space than it did when it was mounted internally. – Christi Oct 12 '13 at 12:27

My guess is the system restore shadow files. The how to geek site explains it:

Why Your Computer Shows the Wrong Amount of Free Space

You’ll probably notice something odd about the amount of free space your hard drive contains, if you look closely. If you right-click your C: drive in Windows, you’ll see a certain amount of space referred to as “Used Space” – in the screenshot below, the hard disk contains 279 GB of files.

However, if you select all the files on your C: drive (including hidden files and Windows system files), right-click them, and select Properties, you’ll notice something odd. The amount of space used by files doesn’t match up with the amount of used space on your hard drive. Here, we have 272 GB worth of files on our C: drive – but Windows is using 279 GB of space. That’s a difference of 7 GB or so – where did all those GBs go?

It turns out that certain types of files don’t appear in Windows Explorer. Files in Windows’ aptly named “shadow storage,” also known as “shadow copies,” don’t appear here. The shadow storage contains System Restore points and previous versions of files for the Previous Versions feature in Windows Explorer.

To view the exact amount of storage used by shadow files on every hard drive attached to your system, you can run the command below. You’ll need to run it as Administrator – to open a Command Prompt window as Administrator, search for Command Prompt in the Start menu, right-click the Command Prompt shortcut, and select Run as administrator.

Run the following command in the Command Prompt window:

vssadmin list shadowstorage

As we can see in the command below, about 9 GB of space is used in our hard drive by the Windows Shadow Copy Storage. The difference above looked more like 7 GB, but that can be explained by rounding.

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This might be plausible, except for the fact that it shows only 72MB of shadow storage used. Moving the drive externally wouldn't delete the shadow storage anyway. – Christi Oct 12 '13 at 12:38
the 1tb drive? well.. now. perhaps it was 50GB when it was used internally. :) Probably (not sure) windows deletes old system restore files if you change the drive cables/ports or enclosure.. I can only guess what happened, this is the most possible scenario I can think of. – medigeek Oct 12 '13 at 12:41
Thanks for trying. When I reached the same point of pretty much having to guess, I decided to ask here and see if anyone knew anything more definite. – Christi Oct 12 '13 at 13:07

A) This is quite commom. Depending on the disk-controller a drive may be reported as having slightly smaller or larger capacity.
There are several ways in which a SATA controller can present the cyclinders/heads/sectors (CHS system) or LBA blocks that the drive contains to the OS. If CHS is used that may result in a smaller capacity than the actual LBA blocks, because CHS numbers need to be rounded down to whole numbers.

E.g Let's assume that CHS = 10/4/5 which amounts to 10*4*5=200 LBA blocks. But if the drive has really 202 blocks those last 2 can't be represented in CHS.

Even if internally everything is done in LBA controllers often still round down to CHS values for compatibility with older Operating Systems that don't support LBA and need CHS in stead. (If the full LBA size is used in such a case the older system would NOT be able to access the last part of the drive.)

In your case the USB3 to SATA bridge in the USB3 enclosure does it slightly differently as the on-board SATA chip in your motherboard. In case of large drives the round-off error may be quite significant as you noticed.

B) Some enclosures reserve a small part of the disk for their build-in encryption and/or PortabelApps software. That may also account for the discrepancy.

Moral of this story: When you move a disk between controllers make sure to re-partition the disk with the geometry as determined by the new controller.
The original partition layout as determined on the original controller may not be valid on the new controller. And you will never know unless you try to access data at the very end of the disk.

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Eh? This makes no sense. The drive will have exactly the same partition table whether it is attached to a SATA port in the PC or plugged in to an external enclosure. To Windows 7 the partition will be the same size wherever you plug it in. This question is about the free space on the existing partition changing, something your answer doesn't even touch on. – Mark Booth Oct 12 '13 at 12:12
@MarkBooth That wasn't clear from your question. I interpreted that as the disk (not partition) appears larger. And my answer stands for that case. And I never said the partition table changed ! But the way the drive is presented to the OS changes which can make the existing partition-table invalid if the new size is smaller than the old one. (Or boundaries of partitions become invalidated by the geometry change. That's another possibility.) – Tonny Oct 12 '13 at 13:12
Where did you get the idea that this was my question @Tonny? Our usernames are completely different! There was no mention of repartitioning in the Q, just that the free space changed. Even if we were talking about repartitioning, the rest of your answer is pure fiction. If you can offer even a single example of a SATA drive which presents a different number of LBA blocks depending on which controller it is plugged into, I will buy you a pint. – Mark Booth Oct 12 '13 at 22:16

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