I am running Ubuntu and move my Dropbox folder from the main SSD the has not enough space to the larger internal HDD. I set all folder permissions to read & write and I am the owner of the folder.
However, Dropbox does not find/use the folder. When I start it says "Your Dropbox folder has been moved or deleted from its original location. Dropbox will not work properly until you move it back. ..." ... and wants me to put it back to the original location.
Did you check that your mounted partition/drive and, in your case, root FS (may be bug but Dropbox application checks the mount point also, in your case!) match Dropbox FS restrictions?
If the problem is unsupported FS, you can take a look here for workaround without changing anything else. Be careful about change in a variable name, noted below the script, which should be corrected by hand (dropboxform bug) before use! DBX_WEB_src have to be modified to DBX_WEB_SRC, that's it.
Hope this gives some direction.
Re: Ubuntu Dropbox Folder Can't Move to Second HDD
In addition to @Здравко's suggestion, it might be that the when you restart your device that the desktop app is starting before the second drive is correctly mounted. This can be a common issue with moving the Dropbox folder to another drive as mentioned in the article that I previously linked.
You should be able to click the "Relink" option and choose to connect it again to the Dropbox folder. However, this issue that you're having can result in some issues with syncing and even potential data loss. Maybe the best way around this would be to disable the Dropbox app from starting on system start up and to manually launch the app once the drive is correctly mounted.
It might be worth just trying the above to see if that will help with this issue even though I understand it's not the most ideal workaround.