Microsoft hasn’t officially released Windows 10’s May 2019 Update to everyone just yet, but the company is already warning of a very weird bug with it. Microsoft is blocking the update on PCs that have a USB hard disk like a thumb drive attached, or machines with an SD Card in the SD Card reader. “Inappropriate drive reassignment can occur on eligible computers that have an external USB device or SD memory card attached during the installation of the May 2019 update,” warns a Microsoft support article.

This means that if you had a USB drive with the D: letter assigned by Windows, after the May 2019 Update it could have shifted along to E: instead. That wouldn’t cause a lot of headaches for most consumers, but for tightly managed business machines any drive letter change could be a big problem. Microsoft is now blocking the May 2019 Update on machines with this storage attached, and all you’ll need to do is remove any USB or SD storage to complete the upgrade.

Microsoft is testing this May update for a longer period of time

Microsoft notes “this issue will be resolved in a future servicing update for Windows 10,” but that clearly won’t be available in time for when the company starts distributing this May 2019 Update later next month. Windows 10 testers on the Release Preview ring can already get access to the final May 2019 Update, and Microsoft is being far more careful with its last-minute testing this time around.

The previous October 2018 Update hit some file deletion issues when it first rolled out, and Microsoft clearly doesn’t want a repeat of any similar issues. Still, it’s strange that Microsoft only detected this USB storage issue late in the May 2019 Update, when it has millions of Windows 10 testers helping flag issues like these.