Summary

Q: How can I specify an external harddrive as the download target for the Mist blockchain?

A: The answer for the Mac version is in Method 1 in In Mist browser how can I move the data (blockchain) to another drive on Mac OSX?. The Linux version is below.

Q: Even if the blockchain data gets corrupted somehow, it won't affect my wallets, right? I can keep them in a folder on my computer instead of the external harddrive, yes or no?

A: Yes. Just make sure that you backup ~/.ethereum/keystore.

Q: I. e. I would prefer having the blockchain data on an external drive and the wallet files on my laptop harddrive. Possible or not?

A: Yes.

The Details

The default subdirectory where your Ethereum data files are stored on Linux is ~/.ethereum .

Within this subdirectory, your blockchain data is stored in the subdirectory chaindata, i.e., ~/.ethereum/chaindata .

Let's assume that your new drive is mounted on /mnt/extdrive1 for this example.

Copy your ~/.ethereum/chaindata subdirectory into your new location /mnt/extdrive1/Ethereum/chaindata using the following command:

mkdir /mnt/extdrive1/Ethereum
cp -rpv ~/.ethereum/chaindata /mnt/extdrive1/Ethereum/

A copy of your Ethereum chaindata subdirectory will now be located in /mnt/extdrive1/Ethereum/chaindata.

Rename your default Ethereum chaindata subdirectory for testing:

mv ~/.ethereum/chaindata ~/.ethereum/OldChaindata_to_delete_when_tested

Soft link your new chaindata subdirectory back to the default location:

ln -s /mnt/extdrive1/Ethereum/chaindata ~/.ethereum/chaindata

Test out your Mist browser and delete your old chaindata directory ONLY AFTER your testing is successful:

rm -rf ~/.ethereum/OldChaindata_to_delete_when_tested

References

• In Mist browser how can I move the data (blockchain) to another drive on Mac OSX?
• ln (Unix)

(1) What's the geth default blockchain sync type when installing via command line tools? Full?

Just tested with geth 1.5.6, the default is full.

(2) Do my settings for geth automatically transfer over to Mist -- and visa versa? (e.g., if I'm running geth --fast will Mist also refer to the same size blockchain?)

Yes, mist uses (in most cases) geth as Ethereum node, so if you run geth --fast, mist will work in fast mode. However, if you stop a geth --fast node, and restart it, it will resume in full mode as far as I remember. This means, after shutting down geth, and starting mist, it will start a full node. But that terminology is misleading in some cases, and you should probably read on here:

• What is Geth's “fast” sync, and why is it faster?

(3) Is it possible to run Mist in "light" mode? I saw from this answer that Mist can be in "full" or "fast" sync but didn't see "light". I tried it, and it looks like I'm not getting any errors yet, but curious if there are known issues.

Light client was just very recently released and you should expect hiccups. The same goes for mist if you use a geth node in light mode. As if Ethereum Stack Exchange isn't awesome already, check out this post:

• What is Geth's “light” sync, and why is it so fast?

(4) Is it possible to have multiple copies of the blockchain on my computer? For example, if I first set up geth using geth -full and then I run geth -light, will the light version overwrite the full version or will I need additional space to sync light?

Yes, that is possible. Running a full node, i.e.,

$ geth             #full node (default),

creates a full copy of the blockchain in ~/.ethereum/geth/chaindata/.

Running a light node, i.e.,

$ geth --light     #,

creates a directory for the state in ~/.ethereum/geth/lightchaindata/. To run both clients at the same time, you need some additional adjustments such as IPC path, ports, etc.

• How to run two nodes on the same device?

However, if you want to run a --fast sync, this only works on the first run of geth. If you already synced the full chain, you will get a message like this if you run geth in fast mode:

I0112 21:09:00.024747 eth/handler.go:119] blockchain not empty, fast sync disabled

If you insist on keeping a full and a fast copy of the blockchain on the same device, you can use the --datadir switch.