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I would like to have an Ubuntu USB I can carry around with me at all times just in case I need/want to use Ubuntu on a computer for some reason. (I find it very useful for a lot of things. Gparted and the dd command I often find useful, for example.)

I tried installing Ubuntu to a USB stick directly, However I found that despite it being a USB 3.0 drive it wasn't fast enough to run a full OS from. Performance was very poor.

So, now I am trying to use a persistent USB drive instead. However, I am unclear how the persistence works and as a result what its limitations are.

How far does persistence extend? I presume it will save settings between boots at the least, but will it save files? If I install something with apt-get will that persist? It seems that when I boot on a MacBook the Live USB lacks the drivers required for WiFi, will drivers persist if I install them?

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Persistence is saved in a file or partition named casper-rw, a persistent home file or partition named home-rw is also possible. A persistent install will save data, program installs, customized desktops, just about everything except some drivers which are loaded before the persistence file is read during boot, (NVidia graphics).

In a syslinux type install, (SDC, Unetbootin, Universal, Rufus, etc), the system resides in the root of the drive as does the casper-rw file. The filesystem for this partition is FAT32 thus maximum size for the persistence file is 4GB.

Prior to 14.04 syslinux type installs could also use a persistent casper-rw and home-rw partition. Persistent partitions no longer work except with grub2 type installs, (mkusb and the original MultiBootUSB).

Mkusb is easy to use, safe, versatile and will make a persistent partition of whatever size you want, it will then use remaining disk space to make a NTFS partition that is visible to both Linux and Windows.

Limitations, well you don't want to unplug a persistent pendrive while data is being written to persistence, luckily mkusb comes with persistence backup tools.

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