Bitcoin wallet.dat file location

my situation is the following: I have a laptop with not much space on disk and therefore using bitcoin on a stick (I start bitcoin-qt with the argument “-datadir=location/to/stick). Now I wanted to transfer some bitcoins from my mobile phone to that account. Accidentaly I typed in a wrong directory (which doesnt exist) and bitcoin-qt lead me to the standard bitcoin directory (which is ~/.bitcoin). I didn’t realize this and transferred the bitcoins to that address.

Now I would like to change the wallet.dat file from the ~/.bitcoin/ directory with the wallet.dat file from the directory on my stick, because my stick has all the blockchain already loaded. Is this possible?

What I have tried so far:

• simply changing the wallet.dat files: when I now start bitcoin-qt it goes in a very very long loop (probably even an endless-loop). It just says “Dursuche erneut…” (german), what means translated: “Researching…”
• doing the above, but with the additional argument “-rescan” (“bitcoin-qt -rescan -datadir=/location/of/stick”): it happens the same as above
• trying to backup the relevant wallet.dat file inside the console of bitcoin-qt: when trying to back it up i get the error: “Error: Wallet backup failed! (code: -4)”