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Depends on the steam library. Is it from another PC? If so, maybe. If not, your boot camp PC won’t see the Mac format drive (To my knowledge, I haven’t used BC in years).
 
Just wondering if I can use my Steam library from an external HDD when booting into Windows 10 via Bootcamp?

Many thanks!
I believe you can if the external drive is configured to be Windows compatible, NTFS or FAT32. You can have both the Steam app and library there, while Windows 10 via Boot Camp partition sits on your internal hard drive. FAT 32 allows you access said drive when running MacOS.
 
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