Microsoft's Surface Pro went on sale today, with new product positioning and and some new official information about storage capacity.

Now that the Surface Pro is on sale, Microsoft has finally disclosed how much free space buyers of the new device can expect. No, those numbers aren't the ones you read last week. Some commentators were horrified when Microsoft mistakenly confirmed that the smaller Surface Pro would only have 23 GB of free disk space and the larger 128 GB model would be limited to 83 GB of storage.

As it turns out, those numbers are way off the mark. The actual figures, when expressed in the decimal system used to calculate the 64 and 128 GB total storage, are 32 GB and 96 GB, respectively. If you use the built-in tool to relocate the Recovery partition, the amount of free space available climbs to 40 GB for the Surface Pro 64 and 104 GB for the Surface Pro 128.

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But Microsoft has botched its messaging to potential buyers.

On its website, under the heading, “What are my storage options?” the company says the Surface Pro 64 GB model leaves “approximately 29 GB available for user content.” The 128 GB model, that page says, has about 89 GB available for personal data.

Both numbers, of course, exclude the Recovery partition, an 8 GB slice of space at the end of the SSD. That partition can be copied to a USB flash drive, increasing the available space to 37 GB and 97 GB, respectively.

A footnote leads to a detailed breakdown of available storage, which was just updated overnight. Confusingly, the numbers describing total storage (64 GB and 128 GB) are listed using the decimal system (billions of bytes). But all of the numbers for used storage space are reported using the Windows binary system, which makes them appear to be smaller than they are. (For a full explanation of the difference, see this post .)

Here’s the breakdown, directly from Microsoft's website, using the Windows binary system:

Category Surface Pro 128 Surface Pro 64 Total storage 119 59 Reserved for recovery tools -9 -9 Windows 8 Pro, built-in apps -21 -21 Available storage, via File Explorer 89 29

*All numbers in GB, as reported by Windows (binary system)

If you convert those numbers to the decimal system, so that they match the 128 and 64 on the box, in advertising, and on the device itself, you get these numbers instead:

Category Surface Pro 128 Surface Pro 64 Total storage 128 64 Reserved for recovery tools -10 -10 Windows 8 Pro, built-in apps -22 -22 Available storage 96 32

*All numbers in billions of bytes, rounded to nearest whole number

Bottom line: With the Surface Pro 128, you get almost exactly 75 percent of the total storage for data files and new apps. With the smaller 64 GB device, you get almost exactly 50 percent for user data and new apps. If you relocate the Recovery partition, those percentages increase to 81 percent and 62 percent, respectively.

Update: Here's a screen shot of the properties for the system partition on a new Surface Pro 128, freshly set up. Notice the decimal value for free space is slightly over 96 billion bytes.

The recovery partition takes up another 8.4 billion bytes. If you relocate it then total free disk space is equal to 104.5 billion bytes.

By the way, the used space on the Surface Pro includes a trial version of Office 2013, which occupies approximately 1.7 GB. If you activate it using a product key or an Office 365 subscription, the amount of free space doesn't change significantly, because the installer files are already present. If you don't want Office, you can uninstall it to reclaim its disk space.

When I checked in with Microsoft late this week, the text in this disclosure was still being reviewed by lawyers. It appears the technical and marketing teams didn’t have much of a say in the process. The footnote at the bottom of the page still mentions only Windows RT; it should also include Windows 8:

Storage size shown is based on decimal system (1 GB = 1 billion bytes) while Windows RT displays disk size using binary system (1 GB = 1,073,741,824 bytes). As a result, 1 GB (in decimal) appears as about 0.93 GB (in binary). Storage capacity is the same; it's just shown differently depending on how you measure a GB (decimal or binary).

Those numbers count only the included solid-state drive, a stock C400 mSATA part from Micron. Both Surface Pro models also include USB 3.0 ports and SD card slots, giving them access to essentially unlimited external storage. (The maximum SD card size is currently 64 GB, but larger cards are in the pipeline.) And of course, both devices are capable of connecting to network and cloud storage as well.

Update: Microsoft has now revised the language in that disclosure. In addition, I can confirm that the Surface Pro includes a trial version of Office 2013 that occupies about 2 GB of disk space. If you install Office, as most buyers probably will, the disk space usage will not increase.

The question for potential buyers, though, is this: How much local data storage do you really need?

Let’s ask the market: As of this morning, a few hours after the Surface Pro went on sale, Microsoft’s web site shows the 128 GB model as out of stock. Would-be buyers can’t even place an order. By contrast, the 64 GB model is available for immediate shipment.

The situation is similar at Microsoft's retail partners, where the 128 GB models were out of stock everywhere I looked. Best Buy isn't taking orders from its website, but Staples is. They can ship a 64 GB model immediately, but you can't place an order for a 128 GB device.

I guess the market has spoken. They prefer at least 128 GB of storage for this "powerful PC in tablet form."

Update: It's worth noting that one model of the Surface RT also sold out early when it went on sale last October. But in that case it was the $499 32 GB model that buyers snapped up quickly. The 64 GB Surface RT continued to be available online. That suggests that buyers of the RT device were price-sensitive and were looking for something with a tablet-like price. By contrast, the initial sell-out of the higher-priced 128 GB Surface Pro suggests early buyers are spec-sensitive and are skeptical of the available storage in the 64 GB device.

In other words, buyers are treating the Surface RT like a tablet and the Surface Pro like a PC.