A USB bootable drive discards all modifications made at shutdown. Is there a way to save it? If possible, how will that affect a version installed on to a different hard disk using that USB drive?
How to make a dedicated Unity USB Stick
Step 1: Download the latest Daily Natty image
Step 2: Create the USB stick installation
When you start the application, it will likely automatically find the ISO you downloaded, which is displayed in the top pane. If you don’t see it, click the Other button and go and select the ISO (typically in your Downloads/ directory in your home directory).
It is recommended you first erase your USB stick, so select it in the bottom pane and click the Erase Disk button. When this is complete, you can then use the slider underneath the Stored in reserved extra space slider to select how much disk space can be used for saving files, documents and other things. Depending on the size of your USB move the slider all the way to the right to use the maximum space available.
Finally, click the Make Startup Disk button and Natty will be installed to your USB stick.
Step 3: Boot into the Key
Step 4: Update and Prepare the Key
Step 5: Test and Report Bugs
I would caution against doing an update or upgrade on a persistent usb drive.
Maximum persistence size is 4GB and this can quickly be fill doing updates.
Upgrades are not possible as the kernel is located in filesystem.squashfs which is read only.
Once the casper-rw file is filled, the drive will not boot,
In this case the casper-rw file may be mounted and data removed from it.
Casper-rw partitions can be used for persistence size over 4GB.