Is there a CD version that lets you boot without changing anything on the HD's? Everything I find is to big to fit a 700MB CD.
I have an old server that crashed and it only has a CD ROM. I've tried using the USB ports but no luck.
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The regular (not the alternate or server) ISOs of major flavors of Ubuntu, whether 32-bit or 64-bit, can all boot live on a machine to test drive without installing.
I personally booted Ubuntu, Xubuntu, Lubuntu from live USB (and in older years, CD) to get a graphical desktop and run some tools, even sometimes on machine without storage.
Most of these ISO are bigger that 700MB. But some Lubuntu ISO are smaller
This link: Lubuntu 14.04.3 LTS (Trusty Tahr) shows the current Lubuntu ISO sorted by size.
You can see that nearly all desktop ISOs of the first release (i386, amd64+mac, amd64) are smaller than 700MB (683MB to 694MB). Only the powerpc variant is over 700MB.
Updated tend to be bigger. So, if 700MB is a hard limit for you, use an ISO of the first release.
That said, since you mention rescuing an old server:
If for your rescue task you can figure out the tools you need, find them use them on your own, then a live Ubuntu (or derivative system) is handy (or any environment you're already familiar with for that matter). This has the benefit that once booted on the machine, even if a needed tool is not on the ISO, one can use
aptitude to get tools from the Internet... assuming the machine can access the Internet. I actually happen to often keep in my work bag some USB sticks with Xubuntu and they saved the day sometimes.
In other cases, e.g. not savvy enough, not sure about the work to be done, cannot access the Internet), it might make sense to use a dedicated rescue live distribution. Being dedicated to a smaller subset of tasks, they can (1) often guide the user, e.g. with a menu (console-based or graphical) to a list of (2) highly relevant tools that wouldn't be included in a generic live system, or (3) tools to fix problems on other widespread operating systems. Also, (4) not running a full desktop may make sense the old target machine doesn't have much RAM (more RAM will be available as e.g. disk cache to boost performance instead of being dedicated to eye candy).
Distrowatch offers a filtering interface that may help find what is best for you. Here's an example: Linux + Data Rescue + Under 700MB distributions according to Distrowatch offers 6 options, including the well-known SystemRescueCD
You might check with OSDisc.com. I don't know how many MB their live/install cd's have, but you could email them and ask. They usually run $5.95 to $19.95 and will refund your money. I'm using a live/install Ubuntu on an old Dell Inspiron laptop to get familiar with the os and it works fine, but it was not purchased from OSDisc.com. Hope this helps.
You can alternatively use a convenient tool called Plop Boot Manager. It’s a small bootloader that provides a menu to select the device you want to boot from. If your BIOS doesn’t support booting from USB devices but still detects the devices at boot, this should be all you need.
(Taken from the screenshots page)
This is Plop’s interface – you just hit the down key to highlight USB and press enter. On some computers I’ve noticed USB devices are displayed as hard drives, so if that doesn’t work be sure to try drives other than the boot drive (typically the first one).
The download contains an ISO image that any CD burning tool will accept. It's only a few megabytes in size.