Posting this in case it helps anyone else. MSFS suddenly started hanging on startup - usually at install manager, or the Continue in normal or safe screen, and occasionally other places early in startup.
No amount of Reboot, Clean Uninstall/Reinstall, logon as a different Xbox id or Windows user, or even install SU10 beta would solve it.
Actual Solution: Unplug my USB SD Card reader! This fixed it like magic.
Additional Symptoms:
Lots of Warnings Event 158 Disk in the Windows System event log, happening every few seconds with the following text
Disk 6 has the same disk identifiers as one or more disks connected to the system. Go to Microsoft’s support website (http://support.microsoft.com) and search for KB2983588 to resolve the issue.
My copy of Adobe Lightroom would not work, it keep seeing a new non-existent card inserted and wanting to start an import. This is what triggered the solution for me as there were no other clues in windows that anything was wrong. Other apps worked fine. The fix worked for this as well.