Unable to remove folder ("not empty" error)

I've tried to follow this but none worked for me when trying to remove the entire folder named BACKUPS from a logical drive which is not mounted automatically nor frequently (it mounts only when I click on the drive icon from the file manager).

I've tried via "sudo", via remounting in read/write mode and also giving all the files the 777 privileges recursively but none of them worked alone, nor in combination with all the others. I tried the sudo rm -rf /media/mano/OS/BACKUP-mint19/Scrivania/BACKUPS command and also via GUI (Caja from MATE) but I get the same error, something similar to: "unable to remove, the folder is not empty" (I'm translating it from Italian).

This is the BACKUPS folder content:

mano@mano-X510UNR:/media/mano/OS/BACKUP-mint19/Scrivania/BACKUPS$ ls -latotale 16drwxrwxrwx 1 mano mano 4096 giu 27  2018  .drwxrwxrwx 1 mano mano 4096 ago 22  2018  ..drwxrwxrwx 1 mano mano 4096 ago 22  2018 'backup 10-4-18'drwxrwxrwx 1 mano mano    0 gen 18  2018 'Backup Asus ZenFone 2 Laser'drwxrwxrwx 1 mano mano    0 mag 24  2018 'Backup Giornaliero'drwxrwxrwx 1 mano mano 4096 mag 26  2018  PenDriveCOELMOdrwxrwxrwx 1 mano mano    0 feb  6  2018 'Pendrive RitFuturo'drwxrwxrwx 1 mano mano    0 giu 12  2018  TETA

Please note that also the BACKUPS folder has the drwxrwxrwx privileges.

This is the fstab content:

# /etc/fstab: static file system information.## Use 'blkid' to print the universally unique identifier for a# device; this may be used with UUID= as a more robust way to name devices# that works even if disks are added and removed. See fstab(5).## <file system> <mount point>   <type>  <options>       <dump>  <pass># / was on /dev/sda5 during installationUUID=9841e28a-ac31-4bce-8b48-1aa93c79a7f4 /               ext4    errors=remount-ro 0       1# /boot/efi was on /dev/sda1 during installationUUID=F4B7-1D7F  /boot/efi       vfat    umask=0077      0       1# swap was on /dev/sda6 during installationUUID=99fcea85-dc92-4449-bc93-b8c9f8256438 none            swap    sw              0       0

This is the part of mtab related to the drive named OS:

/dev/sda3 /media/mano/OS fuseblk rw,relatime,user_id=0,group_id=0,default_permissions,allow_other,blksize=4096 0 0

I just do not understand why it seems nothing is missing (file/folders privileges, rw permission granted when remounted) but still I am not able to remove the content of the BAKUPS folder.

Running Mint 20.1. Any suggestion, please?Thanks.

uname -aLinux mano-X510UNR 5.4.0-91-generic #102-Ubuntu SMP Fri Nov 5 16:31:28 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux