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Author Topic: Wallet Backup Question?  (Read 591 times)
minerfool
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February 19, 2014, 05:29:31 PM
 #1

So, if I backup my wallet.dat on a USB, say with 10 BTC.

Then make some transactions in and out (leftover 9 BTC) , and somehow my PC crashes so need to revert to my saved wallet.dat

What happens when I use this old wallet.dat since it won't include those most recent transactions? will it show the correct 9 BTC?

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Raya
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February 19, 2014, 05:34:07 PM
 #2

It will have 9 Bitcoin on it there is no way you can cheat the system this way otherwise Bitcoin would be worthless.

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February 19, 2014, 05:37:07 PM
 #3

You should see the correct amount of BTC if using Bitcoin-qt, depending to/from which addresses you sent/received BTC.

It depends on the wallet-software.

Bitcoin-qt generates addresses in batches of 100. It will use an address from the pool every time you request a new receiving-address and every time you send coins (for the change-address). So you should backup your wallet-file every 100 (receiving-addresses requested + transactions sent).

Other wallets can have different methods to generate address. For example, Electrum generates addresses deterministically using the initial passphrase. No backups beyond the first are needed here.

See this topic for more information https://bitcointalk.org/index.php?topic=455347.0

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DannyHamilton
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February 19, 2014, 05:44:46 PM
 #4

So, if I backup my wallet.dat on a USB, say with 10 BTC.

Then make some transactions in and out (leftover 9 BTC) , and somehow my PC crashes so need to revert to my saved wallet.dat

What happens when I use this old wallet.dat since it won't include those most recent transactions? will it show the correct 9 BTC?

That depends on how many transactions you sent after you created your backup, what wallet program you are using, and how you have configured that wallet program.

In many cases, it will show the correct 9 BTC.  But with some wallet programs, under some circumstances, it is possible for it to show less.

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February 20, 2014, 07:13:01 AM
 #5

You will see the latest balance according to the blockchain




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marcotheminer
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February 20, 2014, 01:33:32 PM
 #6

Yes it will 1 backup is enough
Baitty
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February 20, 2014, 01:52:15 PM
 #7

You could backup a few times and store the back ups in different places just in case you lose some of them.

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minerfool
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February 20, 2014, 03:46:49 PM
 #8

Yes it will 1 backup is enough

ok great thanks!  i thought I had to backup each time I made transactions or something.

One more question.  Say I backup the wallet.dat with 10 BTC, and simultaneously use a separate PC, download new Bitcoin wallet and replace that with the backed up wallet.dat.

Obviously they cant both show 10BTC balance ... will it error out since that wallet.dat is in use already?


DannyHamilton
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February 20, 2014, 05:01:42 PM
 #9

Yes it will 1 backup is enough

This is not necessarily true.

As I already said:

That depends on how many transactions you sent after you created your backup, what wallet program you are using, and how you have configured that wallet program.

In many cases, it will show the correct 9 BTC.  But with some wallet programs (such as Bitcoin-Qt), under some circumstances (such as after 100 transactions), it is possible for it to show less.

DannyHamilton
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February 20, 2014, 05:08:35 PM
 #10

Yes it will 1 backup is enough

ok great thanks!  i thought I had to backup each time I made transactions or something.

One more question.  Say I backup the wallet.dat with 10 BTC, and simultaneously use a separate PC, download new Bitcoin wallet and replace that with the backed up wallet.dat.

Obviously they cant both show 10BTC balance ... will it error out since that wallet.dat is in use already?

All the value is stored in the blockchain that every full node on the entire network has a full copy of.  The wallet is just a list of private keys (think of them a bit like passwords) that allow you to control some of that value in the blockchain.

If you log into a bank website and create an account, funding that account with $100, and then you log into that same bank account from another computer, it will show the $100 as well, right?

Similar concept here.  Both copies of the wallet will see the same 10 BTC balance.  If you spend some of that 10 BTC with one copy of the wallet, that value will vanish out of the other wallet as well.

This is why a thief doesn't need access to your computer or your wallet to steal your bitcoins.  All they need is the private key of the address that has some unspent bitcoins associated with it.  They can then import that private key into their own wallet and spend your bitcoins.  The bitcoins will vanish from your wallet.

Note that it is a very bad idea to try to run 2 identical copies of the same bitcoin wallet.  The Bitcoin-Qt wallet software will generate new bitcoin addresses that it will keep hidden from you and will send some "change" to these additional hidden addresses when you send a transaction.  Eventually the two wallets will have a different set of "change" addresses.  Then each wallet will show a different balance since they don't have the private keys for the change addresses that were created by the other copy.

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