Is it possible that it may be overwriten because before I manage to recover it I've been able to open it in text file and it was readable wallet.old
It's possible, but it depends on how the HDD is used. If the HDD used to store heavy stuff such as OS, 3D application/game or write lots of files, it's very likely the wallet file is overwritten.
However, if you're serious to recover your wallet, first thing you should do is make RAW copy of your HDD.