Why can't I Unmount my USB HDD?

Troubleshooting
1

This may be related to my post here: https://dietpi.com/forum/t/im-having-trouble-seeing-a-mounted-usb-drive/2459/1 but since I’m not sure, I’m opening a new thread.

This is an RPI 3B with a 2TB external USB drive that has previously worked with DietPi.

When in the Drive_manager, I am unable to unmount my drive. User data, swapfile and RootFS are not on this drive.

Trying to unmount gives a string of text that I can’t read at the bottom, then back to the Drive_manager screen as if nothing happened.

This makes format and “Check & Repair,” options that I can’t use.

What’d I do wrong here?

2 
# mount

Find the /dev/sdX device that is mounted to probably /media/blah/blah_blah_blah or /mnt/blah/blah_blah_blah

Then try sudo umount that /dev device or the /media

if that doesn’t work…try

# sync

then try the umount command