I had both OSs working together, then I did a chown on an external HDD and can not longer boot windows.
Drives: SSD 223 GiB sda1 fat32 142 MiB boot,esp (UEFI Boot) sda2 ext4 37 GiB (/ Root Ubuntu 16.04) sda3 ext4 189GiB (/home Ubuntu 16.04)
HDD 2.73 GiB sdb1 ntfs 600 MiB diag (Win10 RECOVERY) sdb2 fat32 300 MiB boot, esp (Win10 /boot/efi) sdb3 unknown 128 MiB msftres (Win10) sdb4 ntfs 513 GiB msftdata (Win10 Acer - main user partition) sdb5 ntfs 14 GiB diag (Win10 Push Button Reset) sdb6 linux-swap 4 GiB (Linux Swap) sdb7 ntfs 200 Gib msdfdata (*- WinShare) sdb8 ext4 2 TiB (Linux Data Partition)
*- I created the WinShare partition from Windows Disk Manager. In Windows, it only contains D:\Dropbox folder and files. When I set it up, I set up Dropbox in both Windows and Ubu 16.04 to use the same folder. This morning I could access dropbox from either windows or linux and boot back and forth using either the bios change from sda to sdb or by using grub installed on sda.
I plugged in a 1.5 TB USB drive (not listed above, formatted as ext3) to send my backup files to it. In order me to access it in Linux, I had to run "chown buck:buck 1.5TB" (the name of the drive.) I then transferred 500 GB of data from 1.5TB to a folder in sdb8. After that, I booted to a clonezilla USB and backed up sda (disk to file) onto 1.5TB/Backup.
Then I went to boot back into Windows and I get the Repair Windows option.
This morning everything worked. I could boot from the UEFI from either sda or sdb through the bios or I could boot either one from grub on sda.
After moving all the data and performing the backup, I was no longer able to boot to windows by changing the bios or by using grub.
Every time I try to boot, I get a "Repair Windows" screen.