New HDD how to then change computer OS to LL 3.4

[Moltke]

I am wondering once the new HDD is inserted (I’m comfortable with replacing it) do I need to do anything or as long as the boot order is right will my LL 3.4 USB be mounted ok allowing me to simply load and trial LL then install?

Hi [member=7067]JanetBiggar[/member]
If I get this right, you're about to replace a damaged HDD for a new one on this computer, right? If so, then you should have no problems trying LL from the USB key with the new HDD, since there won't be nothing in it due to the fact that it is brand new, and even if it wasn't, trying Linux from a USB key and booting into a live session has nothing to do with the HDD and there should be no interferences whatsoever, as a matter of fact, I'm writing this from a live session of a Linux distro called TuxTrans, which is aimed for translators in the same computer  LL is installed on .

do I need to do anything or as long as the boot order is right will my LL 3.4 USB be mounted ok allowing me to simply load and trial LL then install?

The boot order you're referring to is the one in the bios? If the answer is yes, your best bet is to press F2 which is the key you have to press on that pc in order to access the bios setup and check that it points to the USB, in case it isn't,  simply change it by selecting the USB drive LL is in and that should be it, I'm assuming you know how to do this since you have installed LL before from a USB drive. And of course, after finished trying LL and confirm everything's working as it's supposed to, you'll be able to install it in the new HDD just the same way you've done it before. 
Hope this helps!