How to unmount an external hard drive in linux?

I have an external Toshiba 2TB hard drive with two partitions. The relevant partition is of type BTRFS and mounted in my Parrot OS system. I want to use it for backup purposes but I have a hard time to copy files in Linux for years. For test purposes I copied 4MB to the partition with a glorious speed of 100kB/s. Eventually, Parrot OS told me it is done with. I could also browse the files on the mounted partition. But now I have no idea how to unmount the hard drive without loosing data. Via the GUI nothing happens and also a "sudo umount -l ..." just hangs. It simply keeps Parrot OS busy for hours without anything happening (the external hard drive is flashing). If I unplug the hard drive the copied data is gone when I mount it the next time. Any ideas? It seems like as soon as the hard drive is used (e.g. copy action) it never comes to a rest again. It just keeps flashing and flashing. If I attempt to shutdown my computer it just hangs in the shutdown window or sometimes throws me in a login screen again. **Update**
I forgot to mention that I use a notebook (ASUS UX331U). Yesterday, when I experienced the problems the notebook was not connected to an external power supply. When I connected the notebook yesterday evening and again today, the copy process to the external hard drive of type TOSHIBA Canvio Flex 2TB went as expected. The copy speed was higher than ever and when the system confirmed that the process finished I had no issues whatsoever with unmounting the device. I still do not know what the exact issue is and whether I have to fear some hardware failure of my notebook or the external hard drive anytime soon. Yesterday, I formatted the partition from NTFS to BTRFS. My notebook is often connected to power supply and copying files was already an issue for a long time. However, maybe I was in the unlucky situation that not only one reason was causing issue but a mix of several factors.