Are there drawbacks to encrypting data locally before being encrypted in cloud backup (multiple encryption)? I am planning on backing up my hard drive using a cloud backup service but have become skeptical of the industry based on incidents like these:

http://dbmsmusings.blogspot.com/2011/02/why-i-no-longer-trust-emc.html
https://security.stackexchange.com/questions/7142/online-backup-how-could-encryption-and-de-duplication-be-compatible

And this response to the question of losing one's private key by Altdrive:

"What if I lost my file encryption key?

We don't want to hear that! Your private encryption key is VERY important and personal. Without it, you cannot restore your data in event of a system loss. We highly recommend that when you create your encryption key, you store it in a safe place: e.g. printed out and in a safe deposit box, or backed up somewhere else on the Internet.
If you did, contact us immediately. There may be a few options depending on the circumstances."

Based on lack of trust of these providers, I am planning on encrypting my hard drive using EncFS on an external hard drive and then using Altdrive (the cheapest backup cloud service) to back up that external hard drive, so that regardless of whether I use Altdrive's user-defined private key or their own key, stored on their server, it should not be possible for them to access any of my data.

What are the risks to consider in this approach, eg is there more a chance of corrupting my data since it will be encrypted multiple times, will it be harder to access or obtain individual files, ie maybe I have to download the entire volume since I shouldn't be able to recognize each file on their server (not sure how it works to obtain data from cloud backup providers in general)?

I am using linux.
Apart from the performance hit of encrypting / decrypting twice, and the inconvenience of having two keys to not lose, this should work fine. In fact I wish more people had your approach to data privacy and encrypted things before sending them over the internet!

About Altdrive in particular, a good question to ask yourself about their upload service / upload webpage is: is the data being encrypted on your computer and then uploaded, or uploaded and then encrypted? If their uploader encrypts on your end with a key you never give them, then double-encryption probably isn't adding anything, but if it's sent to them in the clear, then your EncFS idea is good. A good test would be to upload a secret.txt file, then log in to the site from a different computer that doesn't have the encryption key and see if you can download and decrypt the file. If you're not able to decrypt it, then their service is probably trustworthy.

That bit from Altdrive about losing your key is a standard warning in the encryption world: backing up your key protects you from losing it, but means there are more copies of your key for someone to steal. You have to decide which you're more worried about: losing your key, or having it stolen? I can tell you that there are many large corporations and governments who back up decryption keys, and many who don't. You have to decide for yourself.

I've never used EncFS, but from a quick google it looks like it encrypts each file individually, so you should be able to pull down files one-at-a-time rather than having to download the whole block. BUT it looks like it encrypts the filenames by default, so you'll have no idea which file you're grabbing. Does EncFS have a setting to turn off filename encryption?

Again, from a quick google of EncFS:


  Files are encrypted using a volume key, which is stored encrypted in the source directory. A password is used to decrypt this key. wikipedia


You probably want to not upload that volume key file since that would mean that cracking your password gives an attacker your decryption key (and it's ALWAYS easier to crack a password than a full key).