I tried to access the files when the unit displayed as USB but when I tried to acces the main user folder it didn't allow me because it didn't had the rights to do so and ejected the unit.
You seem to know your way arround this things, how do you go about the backup?
You can use the Windows properties to take ownership, however it's a bit flaky and a pain in the neck. There is a registry hack that adds a "take Ownership" dialog to the right click menu that makes it so much easier. However nothing is foolproof with MS, I'm not sure why they make this stuff so hard. The most foolproof way is to boot into a Linux boot disk and copy the files out as it just ignores all the security of Windows. I recommend always keeping a bootable copy of Ubuntu or Mint handy.
As for knowing my way around I get paid to fix Windows.
Here's my personal backup system.
Linux - Kbackup
Mac - BackupList+
Windows - Allway Sync
With all 3 you can dump to an external drive or a shared folder, in my case I dump to a folder on my file server (a headless mini desktop with a large drive). Because these programs only do necessary data, they are a bit more difficult to get started (and restore since that is done manually in most cases) but your backups are much smaller and easily verified. My file server contains my personal documents and such, not the desktop, this way they are shared and I can turn off the desktop at night. Not only does this make file sharing easy between all my systems but actually saves money on my power bill as the server only uses about 15-20 watts of power.
I have 3 stages of backup.
Primary backup - Google drive, this gets the most important, can't be replaced stuff.
Secondary backup - Mega, it has no revisions but it's large and is a good off-site backup. This has harder but not impossible to replace stuff.
External drive - everything else. This has multimedia, desktop/laptop backups, installers, iso's, etc...
I actually have my Google Drive folder inside my Mega folder (which is on the file server) so Mega backs up Google stuff as well, then everything gets backed up to a local external drive. This means my important stuff has 4 copies (laptop, Google, Mega, local external), secondary has 3 (laptop, Mega, local external) and the less important, easily replaced stuff has 2 copies (server and local external). I pay nothing for cloud storage. The computer backups take about 2-3 minutes (I could automate it but meh), the server I just plug in the external and it automatically does it's thing in about 15 minutes, less if I do it frequently, while Google and Mega do theirs 24/7 in the background.
This probably sounds odd and convoluted so here's the why and how.
Google Drive has no Linux client (despite repeatedly claiming they would) and the 3rd party ones are not great except for one you pay for, however Mega's client works great on Linux, Mac or Windows, and since my Google folder is inside my Mega folder I only need to run one client (Mega) on the laptop regardless of what OS it's running (Linux or Mac) to sync both and allow access while on the road (I may switch this to just cloud access rather than having it download local copies soon, sort of like Google Drive on your phone). My file server (Windows) runs both Mega and Google Drive, and my desktop (Linux) runs neither, it just accesses the copies on the file server through the network (as does the laptop while home). This all means that I can reload the laptop, media player or desktop or anything else I'm playing with and not only does all my data remain accessible at all times it also allows them to be OS agnostic and keeps my systems light and easy to backup, usually 2-4gigs at most. Also despite being Windows, the file server is almost entirely immune to viruses because it's headless (no one uses it to browse) and there's no other Windows system on the network to spread an infection to it.