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I know that Ubuntu will not and can not write to a live CD-R/DVD+-R when it is booting from it, as it is a read-only medium. However, the procedure (at least on the data level) for running Ubuntu off an optical disc is different from a USB drive, which is usually write-enabled. if I make a live USB, turn off persistent files, and boot from it, will any data be written to the USB drive (e.g. settings, error logs, temporary files)? Or will Ubuntu just read from it, nothing else?

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USB-Sticks often have a small switch, where you can prevent writing to it altogether - if you don't trust the assertions, made. – user unknown Mar 9 '12 at 0:55
up vote 4 down vote accepted

A live USB acts just like a live CD. Like a live CD, booting is accomplished by extracting a 4 GB filesystem from a highly compressed 700 MB SquashFS image. SquashFS is read-only, so any data that is "changed" in the live session is written to a ramdisk, which (as it's name implies) is kept in RAM and removed on shutdown.

However, you may choose to actually install Ubuntu to a flash drive if you wish. This will cause Ubuntu to treat it as a hard disk, therefore causing it to write to the drive. Note that because Ubuntu needs to be installed on a write-enabled filesystem, you will not be able to use SquashFS and will need the full 5GB (as opposed to ~700 MB) of space required by an uncompressed Ubuntu install.

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So data is written to the drive (causing wear), but removed on shutdown? – InkBlend Mar 3 '12 at 2:49
UH.. no wearing doesn't get removed, it's stay there.. if the PC is accessing the USB stick, it's reading it causing it to wear, wearing does not happens only when it gets written to it.. – Uri Herrera Mar 3 '12 at 4:00
EDIT: What I mean to say, is does data get written to the USB drive (not the hard drive) at any point during a live USB boot? – InkBlend Mar 3 '12 at 17:57
I installed Ubuntu on a USB-Stick some years ago - don't remember which version, but it fit on a 2GB partition. Maybe version 8, 9 or 10. – user unknown Mar 9 '12 at 0:24

I suppose Ubuntu writes data to the USB drive, because it saves the configs we choose. For example, if I install Ubuntu in a computer choosing "Portuguese" as language, the next time I start Ubuntu from a live USB it will use "Portuguese" as the default language.

But I dont know where it writes the data.

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But I wrote that I wanted to know the result if I turned OFF persistent. That is the point of this question: if persistence is turned off, will Ubuntu write any data anyway and why? – InkBlend May 14 '12 at 23:58
Ah, ok, I've misunderstood your question. I haven't tested it yet, I don't know either. – YuriC Jun 13 '12 at 23:21

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