We have found that for external users with a free windows live/onedirve plan, when they are invited to a sharepoint site and click the sync button, it does not sync the document library files.
In testing, if the external user that we have shared a document library with has themselves a full Office 365 plan (like say E3) their onedrive pops up with the document library to sync and all works fine.
However users that do not have a full office 365 plan and instead are using a free windows live type account to access our shared sharepoint document library, the sync button doesn't work.
Today I did a test in a remote session on 3rd party's PC. I opened their onedrive and unlinked their free onedrive account. Then I clicked the sync button in sharepoint and onedrive launched prompting me to sign in. I logged in as a test account that has a full E3 plan - the sync worked perfect.
I then unlinked the test account and clicked the sharepoint library sync button again. Again, I got the prompt to log into onedrive. This time I logged in with the user's free onedrive account only now it just opened up the personal onedrive folder and did not sync the sharepoint library.
So it seems the new onedrive/Sharepoint document library sync only works for external users if the external user has a full office 365 plan - can anyone confirm?
Cheers!