MEMORY CARD IN USE
This was the first problem ever encountered. Whenever i plugged my E71 to PC using the USB cable, i'd get to choose either "Nokia PC Suite" mode or "Mass Media Storage" mode. Whenever i chose Nokia PC Suite mode, i will get an error message saying "F:\ refers to location that is unavailable...bla bla" and therefore i cannot access the memory card. If i choose mass media storage mode, i can access the memory card and carry on with copying stuffs, but when i unplug the phone and browse to my memory card folder, i get the "memory card in use" error in my phone and it stucks there. Only workaround was to restart or reboot my phone and it was really annoying. From forums i browse, some believe that the memory card supplied may have been formatted with a different format which confuses the phone.
SOLUTION: With the memory card inside the phone, format it using the phone from the memory card menu. GOTO Menu > Tools > Memory Card > Format
After formatting, i was able to access my memory card from both nokia pc suite mode and mass media storage mode without the "memory in use" error.
MEMORY CARD CORRUPTED
I had this one only once and i didn't know what had happened. After searching through the internet, some had suggested that the phones randomly encrypt the memory card and therefore had caused the memory card to become unreadable due to the encryption (thus corrupted). The solution below was also suggested from the forum and i myself have not yet any chance to try it (i formatted my memory card and lost all my data when it happened to me that time), will update this post when i do.
SOLUTION: GOTO Menu > Tools > Encryption > Encrypt and save Key > Cancel (prior to actual encryption) unless you wish to encrypt your card.
When you choose 'encrypt and save key', you'll be asked for a passphrase (do not lose this) and a key name. This key name (it should be xxx.pk5 will be saved into folder 'others' in phone memory so be sure to back it up into your PC. The encryption status shall be turned to 'on'.
So whenever you get the memory corrupted error again, just simple go back into encryption mode and use 'encrypt with imported key', key in passphrase and it should solve the issue. Let me know if it doesn't, i may not be able to solve your problem but it would be helpful to other readers.
MEMORY CARD LOCKED
After all the frustrations, i felt like cleaning up my phone so i did a factory reset. As you may have guessed, i got the memory card locked error. Basically when i restarted the phone it would pop up a password menu for the memory card but problem was i didn't set any password, or did i?
SOLUTION: GOTO Menu > Tools > Settings > General > Security > Phone and Sim > Allow remote lock. You will be asked for a remote lock message, i entered "lockit" and then it will ask for lock code, the factory lock code should be "12345" (i strongly suggest you change this).
Thereafter you can simply enter "lockit" (remote lock message) as your memory card password to unlock it.
By configuring the remote lock, it wipes any password of the memory card and replace it with the new remote lock message password.
Another reader, mäenpää suggested to try 'lukitse'as the password which may or may not be a generic/original password for the E71. Try it out and post here is it really works. thanks.
DO NOTE THIS: If you never set "remote lock" option then the above method may just work, but if you had, make sure to try the last remote lock message to unlock your code.