How to create multiboot usb w/ persistence for multiple OS

You have to Convert the ISO to UDRW format using:

hdiutil convert -format UDRW -o destination_file.img source_file.iso

for Further steps and reference click this link,

go with the steps: Create bootable USB stick from ISO in Mac OS X

ISO files cannot be mounted and then written to. ISO 9660 is a read-only file system.

So you'd need to situate a casper-rw file (it's a single file with a filesystem within too) in a location that's physically on the USB drive.

Making a writable filesystem

The Pendrive Linux website shows some details on how to go about creating a casper-rw filesystem. The article is titled: Create a larger casper-rw loop file in Linux. NOTE: A casper-rw filesystem is just a regular filesystem that's been tucked inside a single file. In that example they're using a EXT3 filesystem within it.

Example

1. Make the "casper-rw" image

   $ dd if=/dev/zero of=casper-rw bs=1M count=1024
   1024+0 records in
   1024+0 records out
   1073741824 bytes (1.1 GB) copied, 10.958 s, 98.0 MB/s
   
   $ ls -l casper-rw 
   -rw-rw-r--. 1 saml saml 1073741824 Apr  2 19:56 casper-rw
   

2. Format it as EXT3

   $ mkfs.ext3 -F casper-rw
   mke2fs 1.42.7 (21-Jan-2013)
   Discarding device blocks: done                            
   Filesystem label=
   OS type: Linux
   Block size=4096 (log=2)
   Fragment size=4096 (log=2)
   Stride=0 blocks, Stripe width=0 blocks
   65536 inodes, 262144 blocks
   13107 blocks (5.00%) reserved for the super user
   First data block=0
   Maximum filesystem blocks=268435456
   8 block groups
   32768 blocks per group, 32768 fragments per group
   8192 inodes per group
   Superblock backups stored on blocks: 
       32768, 98304, 163840, 229376
   
   Allocating group tables: done                            
   Writing inode tables: done                            
   Creating journal (8192 blocks): done
   Writing superblocks and filesystem accounting information: done
   

3. Mount it

   $ sudo mount -o loop casper-rw /mnt/
   

4. Check it out

   $ ls /mnt/
   lost+found
   
   $ df -h /mnt/
   Filesystem      Size  Used Avail Use% Mounted on
   /dev/loop0      976M  1.3M  924M   1% /mnt