How can I make a NANDroid backup of an unofficial, unsupported prototype device? (Nvidia Shield Portable 2) - Android Q&A, Help & Troubleshooting

Hello! I've recently come to own an Nvidia Shield portable prototype, and am interested in tinkering with it as part of a long-term project. Before I mess with any of that, I wanted to make a NANDroid backup of the device to make sure I could recover it if I were to mess anything up. However, I'm not really sure how to install TWRP - and, by extension, make a NANDroid backup - onto an unsupported device. There is no official firmware available for this, the only thing I have is the device itself and everything on it.
The device is in complete working condition without any issues, it's just stuck on Android 4.4.2 without any updates available (because it was never officially released). If any further information about the device is needed, please let me know. I am new to these forums, so if there's a more appropriate place to post this let me know and I can post there.
Any help would be appreciated. Thank you!

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Nandroid backups are usually performed in Android's recovery mode - option: backup and restore.

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jwoegerbauer said:
Nandroid backups are usually performed in Android's recovery mode - option: backup and restore.
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Yeah I did manage to access the recovery mode, but it only has stock recovery - no option to backup/restore. I'd need to flash on a custom recovery like TWRP so I can make a NANDroid backup, which brings me back to the problem of not being able to install TWRP on this unsupported prototype.

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To do a full backup by means of ADB's inbuilt backup routine isn't possible. That's fact. Professional apps as Titanium Backup or Online Nandroid Backup requires Android is rooted. Root-free apps as Helium Backup only allows to backup all the videos, pictures, emails, notes, etc. on your Android device.
Possible workaround - NOT TESTED BY ME:
If device's Android supports Linux command tar then you easily can backup device by means of it.
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If tar not present then install BusyBox-NoRoot, obtain path to BusyBox
Example:
Code:
adb devices
adb shell stop
adb shell "<PATH-TO-BUSYBOX> tar -cvf %TEMP%\system.tar /system"
adb shell "<PATH-TO-BUSYBOX> tar -cvf %TEMP%\data.tar /data"
adb shell start

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jwoegerbauer said:
To do a full backup by means of ADB's inbuilt backup routine isn't possible. That's fact. Professional apps as Titanium Backup or Online Nandroid Backup requires Android is rooted. Root-free apps as Helium Backup only allows to backup all the videos, pictures, emails, notes, etc. on your Android device.
Click to expand...
Click to collapse
The device IS rooted but I can't install any of the 'professional' backup apps since they seem to require device-specific configurations.
My question is "how can I do a complete backup of an unofficial, unsupported device?"
The device is rooted and I can access the bootloader, I just don't know how to go about backing it up since it's unsupported

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My understanding of peforming a successful Nandroid backup is that this is completely independent of the hardware / configuration of the device, that it doesn't matter what is the brand / model of the device, that it's totally irrelevant whether OEM/vendor supports the device or not, that it''s only important that device's Android preferredly is rooted so that all partitions / directories can be mounted as RW, that there is enough storage space available ( dozens of GB ) to hold the backup - preferredly on external SD-card. But I may err, as always ...