Wich distro is most suitable to install on USB drive (doesn't touch internal hdd/boot)
 

Hi.

I'd plan to install an fully working Linux distro (i.e. not just a live medium) on a usb drive.

And to make sure that an installer doesn't make the boot sector on the internal hdd/sdd in the computer used to make this install, I need to disconnect the drive while installing.

That lead me to think to ask - is there a list of distros that is safe to install on an usb drive, without touching the main hdd ?

All are safe to do so as long as you can identify which drive you wish to install on during the install process or, as you mention, disconnect the other drive.
You may, however, find that USB2 is slow medium to run an OS from even compared to a spinny drive.

As above - you just have to ensure the boot loader is installed to the USB drive, & that the computer will boot first from USB. :)

I'm assuming you want USB with persistence so that you'll have things you set, update, or install available from boot to boot. Otherwise if you have just a live USB without persistence, then you'll potentially have it boot all the time missing things you wish to use that aren't pre-installed.

I also agree, any distribution is suitable to do this with.

Thank you. But I' unsure how to ensure that.

Say I boot Linux Mint, then how can I ensure that the installer put the boat loader to the usb device I want to install Linux onto (say I use somebody elses computer and cannot disable the internal hdd) ?

You make sure to put it onto the same disk/pendrive that you are installing Linux to. :)

e.g. If you are installing to /dev/sdb, then make sure to put the boot files onto /dev/sdb.

(The internal HDD is often, but not always, /dev/sda.)

fatmac, I appreciate you trying to explain, but I'm not on that level yet so I'm still not getting this.

Anyway - I have this acer Aspire laptop, and I was able to disable the internal ssd and I installed Linux Lite 4.8 onto a Corsair Voyager GT (USB 3.0) 64GB. I'was already informed that I should expect less performance than when installed on internal sata drive. However, it was ridiculously slow when installed onto that thumb drive, so I think that sound as a better idea than it actually are.

Is the laptop equipped with a superspeed (USB 3.0 or higher) port? A USB 3 stick in a USB 2 port still only gets you USB 2 speeds. Which are OK, but much, much slower than a hard disk or internal SSD. Even if the laptop has USB 3 it will probably have a mix of slow, USB 2 and one or two superspeed USB 3 ports.

USB 3 slots are often blue instead of black or white, but don't have to be, and may or may not be marked "SS"

Yes. The laptop is an Acer aspire A315-53-P8XQ and there are one USB3 port at the left side.

I don't know specifically about the Linux Mint installer, but usually it should present you with a list of possible destination drives, together with clues that allow you to identify the drives. The drive's size would be such a clue, or perhaps the installer provides connection details. It's not hard, it just requires extra care, since the installer will do what you tell it to do (e.g. overwrite the hard disk), not what you intend to do.

If you are afraid of overwriting data, and unable to physically disconnect hard disks, perhaps they can be disabled in the computer's BIOS/UEFI.

With pendrive systems, you can often 'load to ram', which takes a while to load up on boot, but will run much faster.