Do I really have to place my wallet file in the data directory? I run bitcoin-core QT and I downloaded the full ledger data to an external HD so I can move between machines. My wallet.dat file is secured someplace else.
I start-up the QT client giving the external hard disk as the data root.
So after I put my real wallet.dat file in the data directory (and restart the client with -rescan):
Do I really have to keep it in the directory where the ledger data is saved? Is that safe?
If you want your wallet database (file) to be in a different directory than your data directory, you can use the -wallet=&lt;path&gt; startup option to specify a custom path to your wallet.
Quoting the manpage:

-wallet=&lt; path &gt;
Specify wallet database path. Can be specified multiple times to
load multiple wallets. Path is interpreted relative to  &lt;wallet‐
dir&gt;  if  it is not absolute, and will be created if it does not
exist (as a directory  containing  a  wallet.dat  file  and  log
files).  For backwards compatibility this will also accept names
of existing data files in .)