First of all try running the wallet A with "-detachdb=1", as psy suggested, and then stop it so you have a consistent database file (wallet) before you copy it over.

If it doesn't work you can use the db4.8_dump and db4.8_load utilities to recover it completely. You may need to install db4.8-util package first.


Import the purged dump again:


Replace wallet.dat with wallet.dat.new, and start bitcoin.