I have a question. Is it possible when you have bitcoin core on your computer (external harddisk) to install another kind of wallet as electrum or so, which needs less space as i read here on other posts.If yes, is it than possible to use the same dat.file as from your bitcoin core wallet?
Yes you can install another wallet software. No you cannot share the wallets across software.
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So i can install for example 'electrum' wallet. But i cannot use my dat.file from my bitcoin core wallet, even if i delete my bitcoin core wallet?
They only possibility than is to send my BTC from my bitcoin address in bitcoin core to a new address in my electrum wallet? Is that correct?