By 'parallel boot', I assume you mean 'dual boot'. Don't be afraid of having a dual boot and using only Linux ;)
Since an Ubuntu flavor takes fewer than 10G, you should consider shrinking your last partition to free enough space for installing Linux. 30G is more than enough to install various IDEs, toolchains, etc.
Then, you can simply mount your 900G partition(s) on Linux to access your data. No loss at all.
Of course, backup your data regularly & especially before the shrink & the installation ;)
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