06-06 23:42 [root@linux MiscData]# dd if=/dev/zero of=testUSB bs=4k count=100000
100000+0 records in\\
100000+0 records out\\
409600000 bytes (410 MB, 391 MiB) copied, 1.27973 s, 320 MB/s\\

  06-06 23:42 [root@linux MiscData]# losetup --find --show testUSB
/dev/loop0\\

  06-06 23:44 [root@linux MiscData]# ls -l testUSB
-rw-r--r-- 1 root root 409600000 Jun  6 23:42 testUSB\\

  06-06 23:44 [root@linux MiscData]# ls -l -h testUSB
-rw-r--r-- 1 root root 391M Jun  6 23:42 testUSB\\

  06-06 23:44 [root@linux MiscData]# mkfs.ext4 /dev/loop0
mke2fs 1.45.6 (20-Mar-2020)\\
Creating filesystem with 400000 1k blocks and 100352 inodes\\
Filesystem UUID: b883ddd9-fa41-48db-a06b-8e077045aba5\\
Superblock backups stored on blocks: \\
	8193, 24577, 40961, 57345, 73729, 204801, 221185\\

Allocating group tables: done   \\                         
Writing inode tables: done   \\                         
Creating journal (8192 blocks): done\\
Writing superblocks and filesystem accounting information: done \\


  06-06 23:45 [root@linux MiscData]# lr /mnt/tmp
total 0\\

  06-06 23:45 [root@linux MiscData]# mount /dev/loop0 /mnt/tmp

  06-06 23:45 [root@linux MiscData]# lr /mnt/tmp
total 12\\
drwx------ 2 root root 12288 Jun  6 23:45 lost+found/\\

  06-06 23:45 [root@linux MiscData]# mount |grep loop0
/dev/loop0 on /mnt/tmp type ext4 (rw,relatime)\\

  06-06 23:45 [root@linux MiscData]# umount /dev/loop0

  06-06 23:46 [root@linux MiscData]# losetup --detach /dev/loop0