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I wanted to bring back an old question, but with a twist. It has been answered, how an SSD could be used as a caching drive for a larger, mechanical drive. What I am looking for is to have the same, but for a drive which is encrypted with TrueCrypt or BitLocker

This question is about windows. I am aware of linux solutions, such as flashcacne and bcache. They all work on a block level, therefore could be used on top of encrypted volume, transparently. When SSD dies, the underlying file system on block device is intact, could be mounted by itself. Very elegant solution, and works very well, but the question is about Windows.

I am not certain about how Windows caching solutions (ExpressCache, DataPlex, etc) work. If SSD is used caching on top of already mounted drive, then encryption is as good as non-existent, because all the files you work with are cached unencrypted on SSD. Has there been any research on possible solutions, where it is verified, that Windows is not writing any plaintext data to SSD?

Maybe, a hardware solution?..

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I don't know how to answer that directly as I have little experience with external SSD cache. OTOH, if you're looking for a faster drive with the price/GB of a mechanical drive I would suggest looking for a Hybrid drive like the Seagate SSHD's. These drives comes with an on-board SSD cache which can be used for write caching and possibly read caching too. Since it works on the disk directly, controlled by the disk firmware, it shows up as a normal drive and everything stored in the SSD cache is encrypted the same way as data making it to the spindles.

On another note, you have to consider that for as long as your computer is powered on (whenever it's on, standby, suspend to ram) the encryption key is kept in RAM and can be retrieved easily with specialized tools. The data is even kept during a full power cycle, and by cooling down the ram modules the data can be retained intact for several minutes without any power. This means even if you machine is protected by secure boot an attacker can move the DIMMs to another computer and boot up a tool to recover the encryption key.

With that in mind, even an unencrypted ssd cache could be considered OK if it's wiped clean during the computer shutdown, in which case the only place that wouldn't be covered would be leaving the computer off after a crash/power failure.

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