in order to mount it you need it to be partitioned and with File System on it. It wasn't mounted. dd is not used on a mounted partition and mkswap doesn't either. And you don't mount a swap a partition.
/dev/sda2 on / type ext4 (rw,relatime)
none on /sys/fs/cgroup/unified type cgroup2 (rw,nosuid,nodev,noexec,relatime,nsdelegate)
/dev/sda1 on /boot/efi type vfat (rw,noatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro)
/dev/sdd1 on /home/ruben/mnt_flash type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro)
/dev/sde1 on /home/ruben/mnt_flash type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro)
[ruben@flatbush ~]$ sudo fdisk -l /dev/sda
Disk /dev/sda: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Disk model: TOSHIBA DT01ACA1
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: CF8DE4F2-CDCA-4949-A2D0-9FFF8EC9E3FB
Device Start End Sectors Size Type
/dev/sda1 4097 618497 614401 300M EFI System
/dev/sda2 618498 1881523267 1880904770 896.9G Linux filesystem
/dev/sda3 1881523268 1953520064 71996797 34.3G Linux swap
Partition 1 does not start on physical sector boundary.
Partition 2 does not start on physical sector boundary.
Partition 3 does not start on physical sector boundary.