Unfortunately, there appears to be a rather odd issue that's preventing some users from upgrading. Essentially, those that attempt to download and install the Windows 10 May 2019 Update with a USB drive or SD card installed, will be presented with an error message that says, "This PC can't be upgraded to Windows 10."
Basically, Microsoft has detected that if an external drive is connected while the update is installed, the various drive letters, both internal and external, can become jumbled. To prevent this from happening to user's devices, Microsoft has simply made it so that the update won't complete with any external drive connected.
The official workaround? Simply unplug all external drives when performing the update and then reconnect them once the update is complete.
A post on ArsT claims that this glitch only affects those upgrading from versions 1803 and 1809 of the operating system while the official status on this issue from Microsoft states that, "This issue will be resolved in a future servicing update for Windows 10. For Windows Insiders, this issue is resolved in build 18877 and later builds."
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