Will your phone run the Windows 10 Technical Preview?

I wanted to show you the Windows 10 Technical Preview for phones, released early yesterday. But the installation process took longer than expected. What went wrong? The answer illustrates the hidden complexity of the global market for mobile devices.

I'm surrounded by mobile phones of all shapes, sizes, and operating systems. Unfortunately, most of the half-dozen Windows Phone devices in my office aren't yet eligible for the preview. And the one device I have at hand that is on the list of supported devices, a five-month-old Lumia 830, stubbornly refused to download the preview build. After installing the Windows Insider app, which is the first prerequisite for unlocking access to the preview, I was greeted with this message:

What makes this Lumia 830 different from others? It's an unlocked international variant, slightly different from the one sold in the United States by AT&T. Although the device I was trying to upgrade appears identical to the one sold by AT&T, mine is an RM-984, which is subtly different from the AT&T variant, RM-983.

That seemingly minor difference isn't unusual. Gabe Aul, who runs the Windows 10 preview program for Microsoft, explained to me yesterday that the seemingly short list of six Lumia devices on the compatibility list for the Windows 10 preview actually includes 704 individual variants, called Phone Operator Pairings or POPs. Each variant is designed for a specific region and mobile operator.

While we were working to stabilize this release, 23 of these 704 POPs have been offered the "Denim" and GDR1 QFE9 update. ... In some cases, during our testing, phones that started on the "Denim" and GDR1 QFE9 baseline result in a problem on first boot of Windows 10 technical preview for phones.

As a result of that potential problem, which could result in a device failing to work after an attempted upgrade, Microsoft chose to block devices with that branch of code installed. That's exactly what happened with the device I was trying to upgrade. It had received the most recent Windows Phone 8.1 update a few days earlier, on February 9. In an email, Gabe explained what to look for:

Devices affected include some Lumia 635, Lumia 730 Dual SIM, and Lumia 830 phones, and can be identified by navigating to Settings > About > (more info) and viewing the OS version. If it shows 8.10.14226.359 then it will not be able to upgrade to this first preview build of Windows 10 for phones. This represents a small set of the overall number of POPs, but devices in the UK and open market unlocked devices have a greater chance of being in this set.

Here are the details from the phone I was trying to upgrade:

Bingo.

With some help from engineers in Redmond, I was able to roll back the QFE9 update, which allowed the Windows 10 Technical Preview to install. Microsoft says it's working on a fix for the problem on first boot that prompted them to block these devices.

Until that fix is available, anyone with a device that is on the compatibility list that refuses to download the preview can try the workaround documented in this forum post.

It's a tedious but straightforward process that involves using the Windows Phone Recovery Tool to roll the device back to a supported configuration, then remaining disconnected from Wi-Fi and disabling automatic updates temporarily (to avoid having the newer build appear and interfere with the preview installation). If you try this workaround, read the instructions carefully and follow them to the letter.

Ironically, anyone who's invested in a flagship Windows Phone device--like the Lumia Icon on Verizon or its global, GSM-based equivalent, the Lumia 930--is ineligible for the preview, at least for now. Likewise, older flagship phones like the Lumia 1520 and 1020 (available on AT&T in the United States) and the Lumia 925 (sold by T-Mobile) are similarly blocked.

The Lumia 830, released last September and pitched as an "affordable flagship" by AT&T in the U.S., is eligible for the preview. The Lumia 830 has a five-inch, 720p display, 16 GB of internal storage (with a MicroSD card slot for expansion), and a 10MP camera. It's currently the most powerful Windows Phone capable of running the Windows 10 code. Because AT&T hasn't yet released the QFE9 update and Denim firmware, those devices are eligible to install the preview.

A handful of low-end Lumia phones are currently the only ones on the supported list besides the 830.

Update: the full list of supported phones is here. At this time it includes only six models, all Microsoft-manufactured Lumia devices: the 830, 730, 630, 635, 636, and 638.

The Lumia 630 and 635 are low-end phones based on the same quad-core Snapdragon 400 CPU and chipset found in the Lumia 830, but have a smaller 4.5-inch display, 512 MB of RAM, and 8 GB of storage (expandable with MicroSD cards). The 635 supports 4G LTE, while the 630 doesn't. Two variants of the 630, the 636 and 638, are also eligible for the Windows 10 preview. They include 1 GB of RAM, support TD-LTE and are intended for the Chinese market.

The only other member of the Windows 10 early-adopter club is the Lumia 730, a dual-SIM, non-LTE version of the 735 "selfie phone." The Lumia 735, which was introduced at the same time as the 730 and 830, doesn't support the preview yet.

The short compatibility list is dictated by development realities: the new Windows is designed to perform in-place upgrades over an existing OS. That requires free space in the OS partition, a luxury that isn't present on most phones that were introduced before last September. Microsoft is working on a feature called "partition stitching" that will allow dynamic adjustments of the OS partition.

When that feature is ready (Microsoft isn't putting a timeframe on that work, but I expect it to be at least a month away), the compatibility list for the Windows 10 preview will expand significantly. But for now it's depressingly short.

Microsoft has been talking up the Windows 10 preview for phones since its big launch event in Redmond in January, and the small but enthusiastic Windows Phone community was eagerly anticipating this week's first preview release. The unexpectedly small list of compatible devices has deflated much of that anticipation.

For now, the best strategy for anyone who wants to test the Windows 10 Technical Preview on a mobile device is to purchase one of those inexpensive Lumia devices, which are available directly from carriers or from third-party sellers. Amazon, for example, has the Lumia 635 at prices ranging from $50 (AT&T) to $80 (T-Mobile). I recommend steering clear of the unlocked Lumia 635 for the time being, unless you want to experience the same frustration I did.