I have 2 laptops: dell 1545 (4gb ram) & 5999 (12gb ram). Issue ONE: The 5999 came with Windows 10; now I have destroyed the Win10 OS twice, trying to burn linux ISOs to a USB hard drives. Not sure how this happens. The Win10 had UEFI boot method and that is no longer the case. Dell Support or Windows support provide a link to download W10 to reinstall. However, I did not get the UEFI BOOT BACK. Issue TWO: Using the 5999, to install Linux, will break the W10 os if I try to install a linux os to my external USB drives. How to prevent destroying W10 OS during install to USB drive? Issue THREE: Using different Linux OS on different HDs with both laptops with USB HDs (user name the same of each drive)...is this a bad practice? On the 1545, the drive bay is left open, each drive OS gets removed and another drive OS will slide into place.

" How to prevent destroying W10 OS during install to USB drive?"
One way.
Remove hard drive. (power or data will do)
Another.
Use any other computer to build what you want that you can remove hard drive. You may have to fool with bios if not uefi computer. Not sure what the state of your system is. I'd see if Dell would provide a recovery disk to return to OEM state.

To some degree, linux will be portable. Not fully portable in all cases and all distro's. Things like video and such may need fooling with. Grub may be an issue. I use a usb flash drive across a number of systems so I know it can work.

There are hundreds of different Linux distributions so the first piece of information to post here would which one(s) you have tried.

This might be a question of terminology but exactly what do you mean by that? Putting a Linux iso on a usb drive or what you refer to as 'burning' to a usb drive will put a Live Linux system on the drive. There are a number of ways to do this and specific software designed to put a Linux distribution on a usb drive to use as an installer. Generally, this is used with flash/pendrives rather than usb hard drives so clarify what you did. If you put the iso on a hard drive, it can be booted if done properly but will be a read only filesystem and you can't make changes to it. Did you actually want to install some Linux to a hard drive to be fully functional?

If your windows was installed UEFI, it is much simpler to install your Linux UEFI also and almost any major Linux system should be able to do that.

The basic information needed is which Linux you tried to install and how you did it. Did you download some Linux and put the iso on a DVD or flash drive and boot it to begin the install and some specifics on what happened. Also, are you familiar with Linux drive/partition naming conventions?

That's what I do. As long as the CPU architecture is the same you can install to portable devices from another machine. Many distros have ways to install them that bypass the installer (which is probably what's nuking your windows OS). I tend towards debootstrap installs for debian based distros like debian or ubuntu. Although that's a very minimal installation. No default passwords, or a bootable kernel installed, until you manually do those steps. There's arch-chroot and other options for other distros.

For slow connections and an already functional linux install, these methods can be very useful. Mostly for the linux in linux part as you have a functional network, and gui environment to get networking firmware, and google quirks before you ever boot the new installation. Plus you can play with more exotic filesystems that the installer might otherwise not offer as a selectable option.

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Okay, my record keeping of procedures are not good. The dell 1545 hd bay has the screws out, so I can slide-in a different harddrive at will. Dell 5999 drive bay is internal and UEFI boot. Currently using on ext harddrives: for MX Linux, Maui, & LXLE. Often I burn the ISO DVD to the harddrive on the Dell 1545 unless it is made for UEFI booting. First time I tried to use the Dell 5999 to burn DVD to ext harddrive I lost my Win10. Probably my fault not enough attention to detail when the installer asked to partition the destination drive and did not notice it was set to the internal drive, where Win10 is located. So, yes I'm familiar with Linux's partition functions. Not so sure about doing the UEFI installs. The attempt to use the 5999 to install FEDORA 26A in UEFI MODE caused something to be written to the internal drive, requiring me to reload Win10. This time I got Win10 installed with the UEFI boot. Not exactly sure how I did it. Maybe leaving the BIOS set to UEFI SECURE made the difference. I have install nearly all versions through the 1545 computer, since most were Legacy booting method. My list of installs: Fedora 26a, Lubuntu, Maui, Korora, Linux Mint, Ubuntu, Manjaro, OpenSuse, Deepin, Antergos, LXLE, Tails, Kali, Bodhi, Gecko, NuTyX, Rosa, Oracle, Chakra, GoboLinux, UbuntuStudio, Mageia, ExTix, Lite, MX, Kwort, Solis. Currently using: Maui, MX & LXLE none are UEFI. Scared to attempt the install of any UEFI Linux OS, it must be done in the Dell 5999 only. I swap ext HD OSs from one machine to the other and wondered if that could bring problems my way? I don't know how to make changes to the hardware configuration file that may be different from one machine to the other. I'm not good using the CLI.

kayakman, please structure your text better.
it will help those trying to read it, but it will also help you structure your thoughts.

just pressing enter twice at suitable intervals (like i just did) already helps a lot.

Sounds to me like you installed over your Windows disk - there are warnings to make sure you have the right disk selected, before you start the installation process. (We've all done it, you just need to be more careful next time.)

A UEFI install should be as easy these days, as any other, the installer should add a boot definition into the EFI drive, so that your Linux can be booted.

You may wish to just use a free virtual machine to run linux from a USB or dedicated hard drive or virtual hard drive set. It would be more portable across the two systems since the basic part of a VM is dedicated hardware model. (cpu will go with system)

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Install linux from ISO file using Rufus in Windows to the USB drive.

EDIT: Sorry, I think I got my software mixed up, Rufus does not do the install, only transfers an ISO to USB and makes it bootable. I got it mixed up with Win2usb since I was using both quite a bit recently.
There are similar softwares for Linux on USB.

Well, there's your problem. Just kidding, sort of.

When I installed Fedora 23 it was simple and GUI. You will need to know a few things like how device names translate to actual hardware. Choose poorly and you wipe out windows (good riddens IMO). You can do a UEFI install on another machine, if risk isn't your forte. As long as the distro is signed by microsoft keys. Fedora and Ubuntu are such critters (in theory). In UEFI you can disable secure boot (depending on your hardware) and boot any distro (with the same CPU arch). Lots of options and installers are not that much of a learning curve these days. YMMV.

Thank you - all of your responses are helpful and I hope to improve my working with linux iso installs.

You are on the right track when mentioning the difference between the Legacy installation and the UEFI installation.
Here is what happens when you install in UEFI mode to a USB hard drive. You set the boot loader to be installed to the external drive (assuming that you knew enough to create a EFI System Partition for the bootloader). The installation program proceeds to ignore that and installs the bootloader to the internal hard drive. WHAT???!!! Yes, sorry, that is just the way UEFI works. Sad but true. It installs the bootloader to the first ESP it finds (on the internal sda) no matter what you want.
So when you try to boot the external drive on a different computer, as you said you wanted to do, you are spitting into the wind trying to install in UEFI mode.
There is another little UEFI trick that will mess you up, even if you managed to manually get the bootloader installed to the USB hard drive. When you install in UEFI mode, some of the boot information is recorded in the computer's NVRAM. When you move the drive to a different computer, that computer does not have the necessary entry in the NVRAM to boot the drive, even if the drive boots on the first computer. Ain't UEFI just a barrel of fun???

laptop thinkpad is running win 7. I want to install unbuntu 17.04 (dual boot) try to install using iso file. Goes through all setup steps but fails to work. Does win 7 use the whole disk? do I have to partition disk to be able to load ubuntu? help

Windows does tend to use the whole disk by default. But you can re-allocate space (if you haven't used the windows OS much) and free up the tail end of the disk for other uses. Something that's been doable, but buried deep in the menus since windows Vista. I just use a USB stick these days for linux, they're fast enough and don't require any fussing with windows. As long as you spam the special key at boot to boot a non-default device. If I tend to not want windows, I just remove the windows drive, problem solved. Boots the only option available every time. Baring that I take an image of the whole drive and zero it out with dd when the drive is small and otherwise soldered to the board.

Windows likes to play with the EFI partition occasionally.
Grab the latest Win 10 iso. Don't install just use the disc to access its command prompt.
https://www.microsoft.com/en-us/soft...d/windows10ISO

Then you gotta do something like this:
https://superuser.com/questions/4607...tloader#504360

Then you have to fix your linux boot loader.