Easiest way to transfer files between hdd and a usb flashdrive?

[xenial]

Hello.
Recently, as some may be aware, I installed Fossapup64 to a usb flashdrive and thanks to the kind help received from members here i managed to get a connection up and running.

Now i have never run an operating system from a usb before and i was wondering what the easiest way would be to transfer some files from sda1 which is the hdd built into my laptop into the usb operating system.I unsuccessfully attempted this earlier and i fell flat on my face and ended up corrupting the fat32 filesytem on the usb.So now i am in the position where fossa64 is reinstalled on the usb but i am not going to attempt any file transfer until i am more confident of what the hell i am doing.

Just another thing....do i need to install another grub boot loader onto the usb.?..obviously the hdd in the laptop has grub but does the usb need a separate grub.?

Thank you.

[bigpup]

If the USB install of Fossapup is booting to a working desktop.
The USB has the boot loader from the Fossapup iso.
The boot loader is already installed on the USB, to boot it.

Each drive has it's own boot loader, as it should be.

Moving files from drive to drive.
unsuccessfully attempted this earlier and i fell flat on my face and ended up corrupting the fat32 filesystem on the usb.

What files are you trying to move or copy?

In general this usually works.

Moving stuff around from different locations.
On the desktop you should have drive icons on the desktop.
Figure out which one is for your internal drive. (sda1)
left mouse click on the sda1 icon to open a Rox file manager window showing it's contents.

Figure out the drive icon for the USB. (probably sdb1)
Do the same thing to have a Rox window showing it's contents.

Now with two Rox windows open.
Navigate in each one to the location in each drive you want to access.
Drag and drop stuff from one Rox window to the other.
A menu will pop up giving option to copy or move.

Warning:
If internal drive has Windows 10 on it.
Windows 10 gets a little troubled when stuff gets added to drive partitions it uses, if Windows 10 did not do it.

If windows 10 is shutdown in hibernate.
It does not fully release control of the drive it is on.
Do full complete shutdown of Windows 10.

[Flash]

In Rox filer, I would open the directory on the source drive that has the file I want to transfer, then open another Rox window of the directory where I want the file to wind up, and drag the file from the source directory to the destination directory. Rox will offer to either move or copy the file to the destination directory.

I use copy-fast to copy audio books from a hard disk to my audio player. It works perfectly.

[xenial]

Thank you flash i shall take that information in hand.Half the problem is finding the wanted files on the chosen partitions.