Hello everyone 
New day, new problem 
Just wanted to start fresh with my new Raspberry Pi 4 and a 375GB WD USB drive.
Installed NCP without problems and went to the web interface (first tried the wizard) to format the connected drive. That apparently worked:
[ nc-format-USB ]
/dev/sda: 8 bytes were erased at offset 0x00000200 (gpt): 45 46 49 20 50 41 52 54
/dev/sda: 8 bytes were erased at offset 0x5752affe00 (gpt): 45 46 49 20 50 41 52 54
/dev/sda: 2 bytes were erased at offset 0x000001fe (PMBR): 55 aa
WARNING: failed to open /dev/btrfs-control, skipping device registration: No such device
Drive sda formatted successfuly and labeled myCloudDrive
What did not work was the mounting of the drive afterwards. That failed through the wizard so I tried to do it manually:
pi@nextcloudpi:/ $ lsblk -f
NAME FSTYPE LABEL UUID FSAVAIL FSUSE% MOUNTPOINT
sda
└─sda1 btrfs myCloudDrive e8064185-b13c-472c-91e0-3565adc70e49
mmcblk0
├─mmcblk0p1 vfat boot 5203-DB74 199.9M 21% /boot
└─mmcblk0p2 ext4 rootfs 2ab3f8e1-7dc6-43f5-b0db-dd5759d51d4e 4.7G 28% /
pi@nextcloudpi:/ $ sudo mkdir /media/USBdrive
pi@nextcloudpi:/ $ sudo mount /dev/sda1 /media/USBdrive
mount: /media/USBdrive: unknown filesystem type 'btrfs'.
Apparently the filesystem of the freshly formatted drive can not be recognized by the Pi. How is that possible? Or is there anything else I missed?
@nachoparker @OliverV